fsck.c:fsck_commit(): use skip_prefix() to verify and skip constant

fsck_commit() uses memcmp() to check if the buffer starts with a
certain prefix, and skips the prefix if it does.

This is exactly what skip_prefix() was designed for.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -281,7 +281,7 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 
 static int fsck_commit(struct commit *commit, fsck_error error_func)
 {
-	const char *buffer = commit->buffer;
+	const char *buffer = commit->buffer, *tmp;
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
 	int parents = 0;
@@ -290,15 +290,17 @@ static int fsck_commit(struct commit *commit, fsck_error error_func)
 	if (commit->date == ULONG_MAX)
 		return error_func(&commit->object, FSCK_ERROR, "invalid author/committer line");
 
-	if (memcmp(buffer, "tree ", 5))
+	buffer = skip_prefix(buffer, "tree ");
+	if (!buffer)
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
-	if (get_sha1_hex(buffer+5, tree_sha1) || buffer[45] != '\n')
+	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')
 		return error_func(&commit->object, FSCK_ERROR, "invalid 'tree' line format - bad sha1");
-	buffer += 46;
-	while (!memcmp(buffer, "parent ", 7)) {
-		if (get_sha1_hex(buffer+7, sha1) || buffer[47] != '\n')
+	buffer += 41;
+	while ((tmp = skip_prefix(buffer, "parent "))) {
+		buffer = tmp;
+		if (get_sha1_hex(buffer, sha1) || buffer[40] != '\n')
 			return error_func(&commit->object, FSCK_ERROR, "invalid 'parent' line format - bad sha1");
-		buffer += 48;
+		buffer += 41;
 		parents++;
 	}
 	graft = lookup_commit_graft(commit->object.sha1);
@@ -322,15 +324,15 @@ static int fsck_commit(struct commit *commit, fsck_error error_func)
 		if (p || parents)
 			return error_func(&commit->object, FSCK_ERROR, "parent objects missing");
 	}
-	if (memcmp(buffer, "author ", 7))
+	buffer = skip_prefix(buffer, "author ");
+	if (!buffer)
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'author' line");
-	buffer += 7;
 	err = fsck_ident(&buffer, &commit->object, error_func);
 	if (err)
 		return err;
-	if (memcmp(buffer, "committer ", strlen("committer ")))
+	buffer = skip_prefix(buffer, "committer ");
+	if (!buffer)
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'committer' line");
-	buffer += strlen("committer ");
 	err = fsck_ident(&buffer, &commit->object, error_func);
 	if (err)
 		return err;