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>
diff --git a/src/buffer.c b/src/buffer.c
index 48e8cb6..fe19269 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2508,6 +2508,7 @@
     free_callback(&buf->b_tsrfu_cb);
 #endif
 #ifdef FEAT_QUICKFIX
+    clear_string_option(&buf->b_p_gefm);
     clear_string_option(&buf->b_p_gp);
     clear_string_option(&buf->b_p_mp);
     clear_string_option(&buf->b_p_efm);