Make :/ accept a regex rather than a fixed pattern

This also makes it trigger anywhere in the commit message, rather than
just at the beginning. Which tends to be a lot more useful.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -679,8 +679,8 @@ static int handle_one_ref(const char *path,
 
 /*
  * This interprets names like ':/Initial revision of "git"' by searching
- * through history and returning the first commit whose message starts
- * with the given string.
+ * through history and returning the first commit whose message matches
+ * the given regular expression.
  *
  * For future extension, ':/!' is reserved. If you want to match a message
  * beginning with a '!', you have to repeat the exclamation mark.
@@ -692,12 +692,17 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 	struct commit_list *list = NULL, *backup = NULL, *l;
 	int retval = -1;
 	char *temp_commit_buffer = NULL;
+	regex_t regex;
 
 	if (prefix[0] == '!') {
 		if (prefix[1] != '!')
 			die ("Invalid search pattern: %s", prefix);
 		prefix++;
 	}
+
+	if (regcomp(&regex, prefix, REG_EXTENDED))
+		die("Invalid search pattern: %s", prefix);
+
 	for_each_ref(handle_one_ref, &list);
 	for (l = list; l; l = l->next)
 		commit_list_insert(l->item, &backup);
@@ -721,12 +726,13 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 		}
 		if (!(p = strstr(p, "\n\n")))
 			continue;
-		if (!prefixcmp(p + 2, prefix)) {
+		if (!regexec(&regex, p + 2, 0, NULL, 0)) {
 			hashcpy(sha1, commit->object.sha1);
 			retval = 0;
 			break;
 		}
 	}
+	regfree(&regex);
 	free(temp_commit_buffer);
 	free_commit_list(list);
 	for (l = backup; l; l = l->next)