Make "parse_commit" return the "struct revision" for the commit.

Also, make it a fatal error to pass in a non-commit object. The callers
never checked, so better check here.

This simplifies merge-base further. It's now so trivial that it's almost
ridiculous.

merge-base.c

@@ -46,10 +46,7 @@ int main(int argc, char **argv)
 	 * That's some time off, though, and in the meantime we know
 	 * that we have a solution to the eventual expense.
 	 */
-	parse_commit(rev1);
-	parse_commit(rev2);
-
-	common = common_parent(lookup_rev(rev1), lookup_rev(rev2));
+	common = common_parent(parse_commit(rev1), parse_commit(rev2));
 	if (!common)
 		die("no common parent found");
 	printf("%s\n", sha1_to_hex(common->sha1));

revision.h

@@ -129,7 +129,7 @@ static unsigned long parse_commit_date(const char *buf)
 	return date;
 }
 
-static int parse_commit(unsigned char *sha1)
+static struct revision * parse_commit(unsigned char *sha1)
 {
 	struct revision *rev = lookup_rev(sha1);
 
@@ -142,7 +142,7 @@ static int parse_commit(unsigned char *sha1)
 		rev->flags |= SEEN;
 		buffer = bufptr = read_sha1_file(sha1, type, &size);
 		if (!buffer || strcmp(type, "commit"))
-			return -1;
+			die("%s is not a commit object", sha1_to_hex(sha1));
 		bufptr += 46; /* "tree " + "hex sha1" + "\n" */
 		while (!memcmp(bufptr, "parent ", 7) && !get_sha1_hex(bufptr+7, parent)) {
 			add_relationship(rev, parent);
@@ -152,7 +152,7 @@ static int parse_commit(unsigned char *sha1)
 		rev->date = parse_commit_date(bufptr);
 		free(buffer);
 	}
-	return 0;	
+	return rev;
 }
 
 #endif /* REVISION_H */