patch 9.1.1396: 'errorformat' is a global option

Problem:  The 'grepformat' option is global option, but it would be
          useful to have it buffer-local, similar to 'errorformat' and
          other quickfix related options (Dani Dickstein)
Solution: Add the necessary code to support global-local 'grepformat',
          allowing different buffers to parse different grep output
          formats (glepnir)

fixes: #17316
closes: #17315

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_quickfix.vim

@@ -2379,6 +2379,25 @@ func Test_grep()
   call s:test_xgrep('l')
 endfunc
 
+func Test_local_grepformat()
+  let save_grepformat = &grepformat
+  set grepformat=%f:%l:%m
+  " The following line are used for the local grep test. Don't remove.
+  " UNIQUEPREFIX:2:3: Local grepformat test
+  new
+  setlocal grepformat=UNIQUEPREFIX:%c:%n:%m
+  call assert_equal('UNIQUEPREFIX:%c:%n:%m', &l:grepformat)
+  call assert_equal('%f:%l:%m', &g:grepformat)
+
+  set grepprg=internal
+  silent grep "^[[:space:]]*\" UNIQUEPREFIX:" test_quickfix.vim
+  call assert_equal(1, len(getqflist()))
+  set grepprg&vim
+
+  bwipe!
+  let &grepformat = save_grepformat
+endfunc
+
 func Test_two_windows()
   " Use one 'errorformat' for two windows.  Add an expression to each of them,
   " make sure they each keep their own state.