revision: add --grep-reflog to filter commits by reflog messages

Similar to --author/--committer which filters commits by author and
committer header fields. --grep-reflog adds a fake "reflog" header to
commit and a grep filter to search on that line.

All rules to --author/--committer apply except no timestamp stripping.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -30,6 +30,7 @@ enum grep_context {
 enum grep_header_field {
 	GREP_HEADER_AUTHOR = 0,
 	GREP_HEADER_COMMITTER,
+	GREP_HEADER_REFLOG,
 
 	/* Must be at the end of the enum */
 	GREP_HEADER_FIELD_MAX