alias: show the call history when an alias is looping

Just printing the command that the user entered is not particularly
helpful when trying to find the alias that causes the loop.

Print the history of substituted commands to help the user find the
offending alias. Mark the entrypoint of the loop with "<==" and the
last command (which looped back to the entrypoint) with "==>".

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

git.c

@@ -675,6 +675,7 @@ static int run_argv(int *argcp, const char ***argv)
 {
 	int done_alias = 0;
 	struct string_list cmd_list = STRING_LIST_INIT_NODUP;
+	struct string_list_item *seen;
 
 	while (1) {
 		/*
@@ -692,9 +693,21 @@ static int run_argv(int *argcp, const char ***argv)
 		/* .. then try the external ones */
 		execv_dashed_external(*argv);
 
-		if (unsorted_string_list_has_string(&cmd_list, *argv[0])) {
+		seen = unsorted_string_list_lookup(&cmd_list, *argv[0]);
+		if (seen) {
+			int i;
+			struct strbuf sb = STRBUF_INIT;
+			for (i = 0; i < cmd_list.nr; i++) {
+				struct string_list_item *item = &cmd_list.items[i];
+
+				strbuf_addf(&sb, "\n  %s", item->string);
+				if (item == seen)
+					strbuf_addstr(&sb, " <==");
+				else if (i == cmd_list.nr - 1)
+					strbuf_addstr(&sb, " ==>");
+			}
 			die(_("alias loop detected: expansion of '%s' does"
-			      " not terminate"), cmd_list.items[0].string);
+			      " not terminate:%s"), cmd_list.items[0].string, sb.buf);
 		}
 
 		string_list_append(&cmd_list, *argv[0]);