alias: add support for aliases of an alias

Aliases can only contain non-alias git commands and their arguments,
not other user-defined aliases.  Resolving further (nested) aliases
is prevented by breaking the loop after the first alias was
processed.  Git then fails with a command-not-found error.

Allow resolving nested aliases by not breaking the loop in
run_argv() after the first alias was processed.  Instead, continue
the loop until `handle_alias()` fails, which means that there are no
further aliases that can be processed.  Prevent looping aliases by
storing substituted commands in `cmd_list` and checking if a command
has been substituted previously.

While we're at it, fix a styling issue just below the added code.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -674,6 +674,7 @@ static void execv_dashed_external(const char **argv)
 static int run_argv(int *argcp, const char ***argv)
 {
 	int done_alias = 0;
+	struct string_list cmd_list = STRING_LIST_INIT_NODUP;
 
 	while (1) {
 		/*
@@ -691,17 +692,25 @@ static int run_argv(int *argcp, const char ***argv)
 		/* .. then try the external ones */
 		execv_dashed_external(*argv);
 
-		/* It could be an alias -- this works around the insanity
+		if (unsorted_string_list_has_string(&cmd_list, *argv[0])) {
+			die(_("alias loop detected: expansion of '%s' does"
+			      " not terminate"), cmd_list.items[0].string);
+		}
+
+		string_list_append(&cmd_list, *argv[0]);
+
+		/*
+		 * It could be an alias -- this works around the insanity
 		 * of overriding "git log" with "git show" by having
 		 * alias.log = show
 		 */
-		if (done_alias)
-			break;
 		if (!handle_alias(argcp, argv))
 			break;
 		done_alias = 1;
 	}
 
+	string_list_clear(&cmd_list, 0);
+
 	return done_alias;
 }