Add TabNew event

TabNew triggers when entering a new tab page, but not when entering an
already created one.

runtime/doc/autocmd.txt

@@ -296,6 +296,7 @@ Name			triggered by ~
 |WinLeave|		before leaving a window
 |TabEnter|		after entering another tab page
 |TabLeave|		before leaving a tab page
+|TabNew|		when creating a new tab page
 |CmdwinEnter|		after entering the command-line window
 |CmdwinLeave|		before leaving the command-line window
 
@@ -859,6 +860,10 @@ TabEnter			Just after entering a tab page. |tab-page|
 TabLeave			Just before leaving a tab page. |tab-page|
 				A WinLeave event will have been triggered
 				first.
+							*TabNew*
+TabNew				When creating a new tab page. |tab-page|
+				After WinEnter and before TabEnter.
+
 							*TermChanged*
 TermChanged			After the value of 'term' has changed.  Useful
 				for re-loading the syntax file to update the

runtime/doc/tabpage.txt

@@ -251,6 +251,9 @@ When switching to another tab page the order is:
 	WinEnter
 	BufEnter
 
+When entering a new tab page (|:tabnew|), TabNew is triggered before TabEnter
+and after WinEnter.
+
 ==============================================================================
 4. Setting 'tabline'					*setting-tabline*
 

runtime/syntax/vim.vim

@@ -76,7 +76,7 @@ syn keyword vimErrSetting contained	hardtabs ht w1200 w300 w9600
 
 " AutoCmd Events {{{2
 syn case ignore
-syn keyword vimAutoEvent contained	BufAdd BufCreate BufDelete BufEnter BufFilePost BufFilePre BufHidden BufLeave BufNew BufNewFile BufRead BufReadCmd BufReadPost BufReadPre BufUnload BufWinEnter BufWinLeave BufWipeout BufWrite BufWriteCmd BufWritePost BufWritePre Cmd-event CmdwinEnter CmdwinLeave ColorScheme CompleteDone CursorHold CursorHoldI CursorMoved CursorMovedI EncodingChanged FileAppendCmd FileAppendPost FileAppendPre FileChangedRO FileChangedShell FileChangedShellPost FileEncoding FileReadCmd FileReadPost FileReadPre FileType FileWriteCmd FileWritePost FileWritePre FilterReadPost FilterReadPre FilterWritePost FilterWritePre FocusGained FocusLost FuncUndefined GUIEnter GUIFailed InsertChange InsertCharPre InsertEnter InsertLeave JobActivity MenuPopup QuickFixCmdPost QuickFixCmdPre QuitPre RemoteReply SessionLoadPost ShellCmdPost ShellFilterPost SourceCmd SourcePre SpellFileMissing StdinReadPost StdinReadPre SwapExists Syntax TabEnter TabLeave TermChanged TermResponse TextChanged TextChangedI User UserGettingBored VimEnter VimLeave VimLeavePre VimResized WinEnter WinLeave 
+syn keyword vimAutoEvent contained	BufAdd BufCreate BufDelete BufEnter BufFilePost BufFilePre BufHidden BufLeave BufNew BufNewFile BufRead BufReadCmd BufReadPost BufReadPre BufUnload BufWinEnter BufWinLeave BufWipeout BufWrite BufWriteCmd BufWritePost BufWritePre Cmd-event CmdwinEnter CmdwinLeave ColorScheme CompleteDone CursorHold CursorHoldI CursorMoved CursorMovedI EncodingChanged FileAppendCmd FileAppendPost FileAppendPre FileChangedRO FileChangedShell FileChangedShellPost FileEncoding FileReadCmd FileReadPost FileReadPre FileType FileWriteCmd FileWritePost FileWritePre FilterReadPost FilterReadPre FilterWritePost FilterWritePre FocusGained FocusLost FuncUndefined GUIEnter GUIFailed InsertChange InsertCharPre InsertEnter InsertLeave JobActivity MenuPopup QuickFixCmdPost QuickFixCmdPre QuitPre RemoteReply SessionLoadPost ShellCmdPost ShellFilterPost SourceCmd SourcePre SpellFileMissing StdinReadPost StdinReadPre SwapExists Syntax TabEnter TabLeave TabNew TermChanged TermResponse TextChanged TextChangedI User UserGettingBored VimEnter VimLeave VimLeavePre VimResized WinEnter WinLeave 
 
 " Highlight commonly used Groupnames {{{2
 syn keyword vimGroup contained	Comment Constant String Character Number Boolean Float Identifier Function Statement Conditional Repeat Label Operator Keyword Exception PreProc Include Define Macro PreCondit Type StorageClass Structure Typedef Special SpecialChar Tag Delimiter SpecialComment Debug Underlined Ignore Error Todo 

src/nvim/ex_docmd.c

@@ -5715,6 +5715,8 @@ void ex_splitview(exarg_T *eap)
     if (win_new_tabpage(cmdmod.tab != 0 ? cmdmod.tab
             : eap->addr_count == 0 ? 0
             : (int)eap->line2 + 1) != FAIL) {
+      apply_autocmds(EVENT_TABNEW, eap->arg, eap->arg,  FALSE, curbuf); 
+      entering_new_tab = true;
       do_exedit(eap, old_curwin);
 
       /* set the alternate buffer for the window we came from */

src/nvim/fileio.c

@@ -5247,6 +5247,7 @@ static struct event_name {
   {"Syntax",          EVENT_SYNTAX},
   {"TabEnter",        EVENT_TABENTER},
   {"TabLeave",        EVENT_TABLEAVE},
+  {"TabNew",          EVENT_TABNEW},
   {"TermChanged",     EVENT_TERMCHANGED},
   {"TermResponse",    EVENT_TERMRESPONSE},
   {"TextChanged",     EVENT_TEXTCHANGED},

src/nvim/fileio.h

@@ -95,6 +95,7 @@ typedef enum auto_event {
   EVENT_CURSORMOVEDI,           /* cursor was moved in Insert mode */
   EVENT_TABLEAVE,               /* before leaving a tab page */
   EVENT_TABENTER,               /* after entering a tab page */
+  EVENT_TABNEW,                 /* when creating a new tab */
   EVENT_SHELLCMDPOST,           /* after ":!cmd" */
   EVENT_SHELLFILTERPOST,        /* after ":1,2!cmd", ":w !cmd", ":r !cmd". */
   EVENT_TEXTCHANGED,            /* text was modified */

test/functional/autocmd/tabnew_spec.lua

@@ -0,0 +1,24 @@
+local helpers = require('test.functional.helpers')
+local clear, nvim, buffer, curbuf, curwin, eq, neq, ok =
+  helpers.clear, helpers.nvim, helpers.buffer, helpers.curbuf, helpers.curwin,
+  helpers.eq, helpers.neq, helpers.ok
+
+describe('TabNew', function()
+    describe('au TabNew', function()
+        clear()
+        describe('with * as <afile>', function()
+            it('matches when opening any new tab', function()
+                nvim('command', 'au! TabNew * echom "tabnew:".tabpagenr().":".bufnr("")')
+                eq("\ntabnew:2:1", nvim('command_output', 'tabnew'))
+                eq("\ntabnew:3:2\n\"test.x\" [New File]", nvim('command_output', 'tabnew test.x'))
+            end)
+        end)
+        describe('with FILE as <afile>', function()
+            it('matches when opening a new tab for FILE', function()
+                tmp_path = nvim('eval', 'tempname()')
+                nvim('command', 'au! TabNew '..tmp_path..' echom "tabnew:match"')
+                eq("\ntabnew:4:3\ntabnew:match\n\""..tmp_path.."\" [New File]", nvim('command_output', 'tabnew '..tmp_path))
+           end)
+        end)
+    end)
+end)