Make "git-remote prune" delete refs according to fetch specs

A remote may be configured to fetch into tracking branches that
do not match the remote name.  For example a user may have created
extra remotes that will fetch to the same tracking branch namespace,
but from different URLs:

  [remote "origin"]
    url = git://git.kernel.org/pub/scm/git/git.git
    fetch = refs/heads/*:refs/remotes/origin/*

  [remote "alt"]
    url = git://repo.or.cz/alt-git.git
    fetch = refs/heads/*:refs/remotes/origin/*

When running `git remote prune alt` we expect stale branches to
be removed from "refs/remotes/origin/*" and not from the unused
namespace of "refs/remotes/alt/*".
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -419,27 +419,10 @@ static int show_or_prune(int argc, const char **argv, int prune)
 					states.remote->name);
 
 		if (prune) {
-			struct strbuf buf;
-			int prefix_len;
-
-			strbuf_init(&buf, 0);
-			if (states.remote->fetch_refspec_nr == 1 &&
-					states.remote->fetch->pattern &&
-					!strcmp(states.remote->fetch->src,
-						states.remote->fetch->dst))
-				/* handle --mirror remote */
-				strbuf_addstr(&buf, "refs/heads/");
-			else
-				strbuf_addf(&buf, "refs/remotes/%s/", *argv);
-			prefix_len = buf.len;
-
 			for (i = 0; i < states.stale.nr; i++) {
-				strbuf_setlen(&buf, prefix_len);
-				strbuf_addstr(&buf, states.stale.items[i].path);
-				result |= delete_ref(buf.buf, NULL);
+				const char *refname = states.stale.items[i].util;
+				result |= delete_ref(refname, NULL);
 			}
-
-			strbuf_release(&buf);
 			goto cleanup_states;
 		}
 

t/t5505-remote.sh

@@ -164,6 +164,24 @@ test_expect_success 'add --mirror && prune' '
 	 git rev-parse --verify refs/heads/side)
 '
 
+test_expect_success 'add alt && prune' '
+	(mkdir alttst &&
+	 cd alttst &&
+	 git init &&
+	 git remote add -f origin ../one &&
+	 git config remote.alt.url ../one &&
+	 git config remote.alt.fetch "+refs/heads/*:refs/remotes/origin/*") &&
+	(cd one &&
+	 git branch -m side side2) &&
+	(cd alttst &&
+	 git rev-parse --verify refs/remotes/origin/side &&
+	 ! git rev-parse --verify refs/remotes/origin/side2 &&
+	 git fetch alt &&
+	 git remote prune alt &&
+	 ! git rev-parse --verify refs/remotes/origin/side &&
+	 git rev-parse --verify refs/remotes/origin/side2)
+'
+
 cat > one/expect << EOF
   apis/master
   apis/side