prune_remote(): use delete_refs()

This slightly changes how errors are reported. The old and new code
both report errors that come from repack_without_refs() the same way.
But if an error occurs within delete_ref(), the old version only
emitted an error within delete_ref() without further comment. The new
version (in delete_refs()) still emits that error, but then follows it
up with

    error(_("could not remove reference %s"), refname)

The "could not remove reference" error originally came from a similar
message in remove_branches() (from builtin/remote.c).

This is an improvement, because the error from delete_ref() (which
usually comes from ref_transaction_commit()) can be pretty low-level,
like

    Cannot lock ref '%s': unable to resolve reference %s: %s

where the last "%s" is the original strerror.

In any case, I don't think we need to sweat the details too much,
because these errors should only ever be seen in the case of a broken
repository or a race between two processes; i.e., only in pretty rare
and anomalous situations.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/remote.c

@@ -1314,19 +1314,12 @@ static int prune_remote(const char *remote, int dry_run)
 		string_list_append(&refs_to_prune, item->util);
 	string_list_sort(&refs_to_prune);
 
-	if (!dry_run) {
-		struct strbuf err = STRBUF_INIT;
-		if (repack_without_refs(&refs_to_prune, &err))
-			result |= error("%s", err.buf);
-		strbuf_release(&err);
-	}
+	if (!dry_run)
+		result |= delete_refs(&refs_to_prune);
 
 	for_each_string_list_item(item, &states.stale) {
 		const char *refname = item->util;
 
-		if (!dry_run)
-			result |= delete_ref(refname, NULL, 0);
-
 		if (dry_run)
 			printf_ln(_(" * [would prune] %s"),
 			       abbrev_ref(refname, "refs/remotes/"));