remote prune: print the list of pruned branches

This command is really too quiet which make it unconfortable to use.

Also implement a --dry-run option, in place of the original -n one, to
list stale tracking branches that will be pruned, but do not actually
prune them.

Add a test case for --dry-run.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 'git-remote' add [-t <branch>] [-m <master>] [-f] [--mirror] <name> <url>
 'git-remote' rm <name>
 'git-remote' show [-n] <name>
-'git-remote' prune <name>
+'git-remote' prune [-n | --dry-run] <name>
 'git-remote' update [group]
 
 DESCRIPTION
@@ -80,9 +80,8 @@ These stale branches have already been removed from the remote repository
 referenced by <name>, but are still locally available in
 "remotes/<name>".
 +
-With `-n` option, the remote heads are not confirmed first with `git
-ls-remote <name>`; cached information is used instead.  Use with
-caution.
+With `--dry-run` option, report what branches will be pruned, but do no
+actually prune them.
 
 'update'::
 

builtin-remote.c

@@ -523,10 +523,10 @@ static int show(int argc, const char **argv)
 
 static int prune(int argc, const char **argv)
 {
-	int no_query = 0, result = 0;
+	int dry_run = 0, result = 0;
 	struct option options[] = {
 		OPT_GROUP("prune specific options"),
-		OPT_BOOLEAN('n', NULL, &no_query, "do not query remotes"),
+		OPT__DRY_RUN(&dry_run),
 		OPT_END()
 	};
 	struct ref_states states;
@@ -540,11 +540,23 @@ static int prune(int argc, const char **argv)
 	for (; argc; argc--, argv++) {
 		int i;
 
-		get_remote_ref_states(*argv, &states, !no_query);
+		get_remote_ref_states(*argv, &states, 1);
+
+		printf("Pruning %s\n", *argv);
+		if (states.stale.nr)
+			printf("URL: %s\n",
+			       states.remote->url_nr
+			       ? states.remote->url[0]
+			       : "(no URL)");
 
 		for (i = 0; i < states.stale.nr; i++) {
 			const char *refname = states.stale.items[i].util;
-			result |= delete_ref(refname, NULL);
+
+			if (!dry_run)
+				result |= delete_ref(refname, NULL);
+
+			printf(" * [%s] %s\n", dry_run ? "would prune" : "pruned",
+			       skip_prefix(refname, "refs/remotes/"));
 		}
 
 		/* NEEDSWORK: free remote */

t/t5505-remote.sh

@@ -165,6 +165,24 @@ test_expect_success 'prune' '
 	 ! git rev-parse refs/remotes/origin/side)
 '
 
+cat > test/expect << EOF
+Pruning origin
+URL: $(pwd)/one/.git
+ * [would prune] origin/side2
+EOF
+
+test_expect_success 'prune --dry-run' '
+	(cd one &&
+	 git branch -m side2 side) &&
+	(cd test &&
+	 git remote prune --dry-run origin > output &&
+	 git rev-parse refs/remotes/origin/side2 &&
+	 ! git rev-parse refs/remotes/origin/side &&
+	(cd ../one &&
+	 git branch -m side side2) &&
+	 test_cmp expect output)
+'
+
 test_expect_success 'add --mirror && prune' '
 	(mkdir mirror &&
 	 cd mirror &&