vim-patch:8.1.1498: ":write" increments b:changedtick even though nothing changed

Problem:    ":write" increments b:changedtick even though nothing changed.
            (Daniel Hahler)
Solution:   Only increment b:changedtick if the modified flag is reset.
c024b46678

runtime/doc/eval.txt

@@ -1313,8 +1313,10 @@ One local buffer variable is predefined:
 					*b:changedtick* *changetick*
 b:changedtick	The total number of changes to the current buffer.  It is
 		incremented for each change.  An undo command is also a change
-		in this case.  This can be used to perform an action only when
+		in this case.  Resetting 'modified' when writing the buffer is
-		the buffer has changed.  Example: >
+		also counted.
+		This can be used to perform an action only when the buffer has
+		changed.  Example: >
 		    :if my_changedtick != b:changedtick
 		    :	let my_changedtick = b:changedtick
 		    :	call My_Update()

src/nvim/buffer.c

@@ -144,7 +144,7 @@ read_buffer(
     if (!readonlymode && !BUFEMPTY()) {
       changed();
     } else if (retval != FAIL) {
-      unchanged(curbuf, false);
+      unchanged(curbuf, false, true);
     }
 
     apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, false,
@@ -299,7 +299,7 @@ int open_buffer(
       || (aborting() && vim_strchr(p_cpo, CPO_INTMOD) != NULL)) {
     changed();
   } else if (retval != FAIL && !read_stdin && !read_fifo) {
-    unchanged(curbuf, false);
+    unchanged(curbuf, false, true);
   }
   save_file_ff(curbuf);                 // keep this fileformat
 
@@ -641,13 +641,11 @@ void close_buffer(win_T *win, buf_T *buf, int action, int abort_if_last)
   }
 }
 
-/*
+/// Make buffer not contain a file.
- * Make buffer not contain a file.
- */
 void buf_clear_file(buf_T *buf)
 {
   buf->b_ml.ml_line_count = 1;
-  unchanged(buf, true);
+  unchanged(buf, true, true);
   buf->b_p_eol = true;
   buf->b_start_eol = true;
   buf->b_p_bomb = false;

src/nvim/change.c

@@ -478,9 +478,11 @@ changed_lines(
   }
 }
 
-/// Called when the changed flag must be reset for buffer "buf".
+/// Called when the changed flag must be reset for buffer `buf`.
-/// When "ff" is true also reset 'fileformat'.
+/// When `ff` is true also reset 'fileformat'.
-void unchanged(buf_T *buf, int ff)
+/// When `always_inc_changedtick` is true b:changedtick is incremented even
+/// when the changed flag was off.
+void unchanged(buf_T *buf, int ff, bool always_inc_changedtick)
 {
   if (buf->b_changed || (ff && file_ff_differs(buf, false))) {
     buf->b_changed = false;
@@ -491,8 +493,10 @@ void unchanged(buf_T *buf, int ff)
     check_status(buf);
     redraw_tabline = true;
     need_maketitle = true;  // set window title later
+    buf_inc_changedtick(buf);
+  } else if (always_inc_changedtick) {
+    buf_inc_changedtick(buf);
   }
-  buf_inc_changedtick(buf);
 }
 
 /// Insert string "p" at the cursor position.  Stops at a NUL byte.

src/nvim/ex_cmds2.c

@@ -1323,7 +1323,7 @@ void dialog_changed(buf_T *buf, bool checkall)
       (void)buf_write_all(buf, false);
     }
   } else if (ret == VIM_NO) {
-    unchanged(buf, true);
+    unchanged(buf, true, false);
   } else if (ret == VIM_ALL) {
     // Write all modified files that can be written.
     // Skip readonly buffers, these need to be confirmed
@@ -1348,7 +1348,7 @@ void dialog_changed(buf_T *buf, bool checkall)
   } else if (ret == VIM_DISCARDALL) {
     // mark all buffers as unchanged
     FOR_ALL_BUFFERS(buf2) {
-      unchanged(buf2, true);
+      unchanged(buf2, true, false);
     }
   }
 }

src/nvim/fileio.c

@@ -3485,10 +3485,10 @@ restore_backup:
   if (reset_changed && whole && !append
       && !write_info.bw_conv_error
       && (overwriting || vim_strchr(p_cpo, CPO_PLUS) != NULL)) {
-    unchanged(buf, true);
+    unchanged(buf, true, false);
     const varnumber_T changedtick = buf_get_changedtick(buf);
     if (buf->b_last_changedtick + 1 == changedtick) {
-      // changedtick is always incremented in unchanged() but that
+      // b:changedtick may be incremented in unchanged() but that
       // should not trigger a TextChanged event.
       buf->b_last_changedtick = changedtick;
     }
@@ -5107,14 +5107,14 @@ void buf_reload(buf_T *buf, int orig_mode)
         }
         (void)move_lines(savebuf, buf);
       }
-    } else if (buf == curbuf) {  /* "buf" still valid */
+    } else if (buf == curbuf) {  // "buf" still valid.
-      /* Mark the buffer as unmodified and free undo info. */
+      // Mark the buffer as unmodified and free undo info.
-      unchanged(buf, TRUE);
+      unchanged(buf, true, true);
       if ((flags & READ_KEEP_UNDO) == 0) {
         u_blockfree(buf);
         u_clearall(buf);
       } else {
-        /* Mark all undo states as changed. */
+        // Mark all undo states as changed.
         u_unchanged(curbuf);
       }
     }

src/nvim/memline.c

@@ -1003,7 +1003,7 @@ void ml_recover(void)
     set_option_value("fenc", 0L, (char *)b0_fenc, OPT_LOCAL);
     xfree(b0_fenc);
   }
-  unchanged(curbuf, TRUE);
+  unchanged(curbuf, true, true);
 
   bnum = 1;             /* start with block 1 */
   page_count = 1;       /* which is 1 page */

src/nvim/undo.c

@@ -2294,7 +2294,7 @@ static void u_undoredo(int undo, bool do_buf_event)
   if (old_flags & UH_CHANGED) {
     changed();
   } else {
-    unchanged(curbuf, FALSE);
+    unchanged(curbuf, false, true);
   }
 
   // because the calls to changed()/unchanged() above will bump changedtick