pretty: avoid reading past end-of-string with "%G"

If the user asks for --format=%G with nothing else, we
correctly realize that "%G" is not a valid placeholder (it
should be "%G?", "%GK", etc). But we still tell the
strbuf_expand code that we consumed 2 characters, causing it
to jump over the trailing NUL and output garbage.

This also fixes the case where "%GX" would be consumed (and
produce no output). In other cases, we pass unrecognized
placeholders through to the final string.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/pretty.c b/pretty.c
index 3c43db558a..fe249f8820 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1267,6 +1267,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->signature_check.key)
 				strbuf_addstr(sb, c->signature_check.key);
 			break;
+		default:
+			return 0;
 		}
 		return 2;
 	}