patch 8.0.1831: sometimes the quickfix title is incorrectly prefixed with ':'

Problem:    Sometimes the quickfix title is incorrectly prefixed with ':'.
Solution:   Prepend the colon in another way. (Yegappan Lakshmanan, closes
            #2905)

src/testdir/test_quickfix.vim

@@ -3250,3 +3250,96 @@ func Test_shorten_fname()
   silent! clist
   call assert_equal('test_quickfix.vim', bufname('test_quickfix.vim'))
 endfunc
+
+" Quickfix title tests
+" In the below tests, 'exe "cmd"' is used to invoke the quickfix commands.
+" Otherwise due to indentation, the title is set with spaces at the beginning
+" of the command.
+func Test_qftitle()
+  call writefile(["F1:1:Line1"], 'Xerr')
+
+  " :cexpr
+  exe "cexpr readfile('Xerr')"
+  call assert_equal(":cexpr readfile('Xerr')", getqflist({'title' : 1}).title)
+
+  " :cgetexpr
+  exe "cgetexpr readfile('Xerr')"
+  call assert_equal(":cgetexpr readfile('Xerr')",
+					\ getqflist({'title' : 1}).title)
+
+  " :caddexpr
+  call setqflist([], 'f')
+  exe "caddexpr readfile('Xerr')"
+  call assert_equal(":caddexpr readfile('Xerr')",
+					\ getqflist({'title' : 1}).title)
+
+  " :cbuffer
+  new Xerr
+  exe "cbuffer"
+  call assert_equal(':cbuffer (Xerr)', getqflist({'title' : 1}).title)
+
+  " :cgetbuffer
+  edit Xerr
+  exe "cgetbuffer"
+  call assert_equal(':cgetbuffer (Xerr)', getqflist({'title' : 1}).title)
+
+  " :caddbuffer
+  call setqflist([], 'f')
+  edit Xerr
+  exe "caddbuffer"
+  call assert_equal(':caddbuffer (Xerr)', getqflist({'title' : 1}).title)
+
+  " :cfile
+  exe "cfile Xerr"
+  call assert_equal(':cfile Xerr', getqflist({'title' : 1}).title)
+
+  " :cgetfile
+  exe "cgetfile Xerr"
+  call assert_equal(':cgetfile Xerr', getqflist({'title' : 1}).title)
+
+  " :caddfile
+  call setqflist([], 'f')
+  exe "caddfile Xerr"
+  call assert_equal(':caddfile Xerr', getqflist({'title' : 1}).title)
+
+  " :grep
+  set grepprg=internal
+  exe "grep F1 Xerr"
+  call assert_equal(':grep F1 Xerr', getqflist({'title' : 1}).title)
+
+  " :grepadd
+  call setqflist([], 'f')
+  exe "grepadd F1 Xerr"
+  call assert_equal(':grepadd F1 Xerr', getqflist({'title' : 1}).title)
+  set grepprg&vim
+
+  " :vimgrep
+  exe "vimgrep F1 Xerr"
+  call assert_equal(':vimgrep F1 Xerr', getqflist({'title' : 1}).title)
+
+  " :vimgrepadd
+  call setqflist([], 'f')
+  exe "vimgrepadd F1 Xerr"
+  call assert_equal(':vimgrepadd F1 Xerr', getqflist({'title' : 1}).title)
+
+  call setqflist(['F1:10:L10'], ' ')
+  call assert_equal(':setqflist()', getqflist({'title' : 1}).title)
+
+  call setqflist([], 'f')
+  call setqflist(['F1:10:L10'], 'a')
+  call assert_equal(':setqflist()', getqflist({'title' : 1}).title)
+
+  call setqflist([], 'f')
+  call setqflist(['F1:10:L10'], 'r')
+  call assert_equal(':setqflist()', getqflist({'title' : 1}).title)
+
+  close
+  call delete('Xerr')
+
+  call setqflist([], ' ', {'title' : 'Errors'})
+  copen
+  call assert_equal('Errors', w:quickfix_title)
+  call setqflist([], 'r', {'items' : [{'filename' : 'a.c', 'lnum' : 10}]})
+  call assert_equal('Errors', w:quickfix_title)
+  cclose
+endfunc