vim-patch:8.1.0311: filtering entries in a quickfix list is not easy

Problem:    Filtering entries in a quickfix list is not easy.
Solution:   Add the cfilter plugin. (Yegappan Lakshmanan)
8c5e0093c9

runtime/pack/dist/opt/cfilter/plugin/cfilter.vim

@@ -0,0 +1,43 @@
+" cfilter.vim: Plugin to filter entries from a quickfix/location list
+" Last Change: 	May 12, 2018
+" Maintainer: 	Yegappan Lakshmanan (yegappan AT yahoo DOT com)
+" Version:	1.0
+"
+" Commands to filter the quickfix list:
+"   :Cfilter[!] {pat}
+"       Create a new quickfix list from entries matching {pat} in the current
+"       quickfix list. Both the file name and the text of the entries are
+"       matched against {pat}. If ! is supplied, then entries not matching
+"       {pat} are used.
+"   :Lfilter[!] {pat}
+"       Same as :Cfilter but operates on the current location list.
+"
+if exists("loaded_cfilter")
+    finish
+endif
+let loaded_cfilter = 1
+
+func s:Qf_filter(qf, pat, bang)
+    if a:qf
+	let Xgetlist = function('getqflist')
+	let Xsetlist = function('setqflist')
+	let cmd = ':Cfilter' . a:bang
+    else
+	let Xgetlist = function('getloclist', [0])
+	let Xsetlist = function('setloclist', [0])
+	let cmd = ':Lfilter' . a:bang
+    endif
+
+    if a:bang == '!'
+	let cond = 'v:val.text !~# a:pat && bufname(v:val.bufnr) !~# a:pat'
+    else
+	let cond = 'v:val.text =~# a:pat || bufname(v:val.bufnr) =~# a:pat'
+    endif
+
+    let items = filter(Xgetlist(), cond)
+    let title = cmd . ' ' . a:pat
+    call Xsetlist([], ' ', {'title' : title, 'items' : items})
+endfunc
+
+com! -nargs=+ -bang Cfilter call s:Qf_filter(1, <q-args>, <q-bang>)
+com! -nargs=+ -bang Lfilter call s:Qf_filter(0, <q-args>, <q-bang>)