remote: deprecate --mirror

The configuration created by plain --mirror is dangerous and
useless, and we now have --mirror=fetch and --mirror=push to
replace it. Let's warn the user.

One alternative to this is to try to guess which type the
user wants. In a non-bare repository, a fetch mirror doesn't
make much sense, since it would overwrite local commits. But
in a bare repository, you might use either type, or even
both (e.g., if you are acting as an intermediate drop-point
across two disconnected networks).

So rather than try for complex heuristics, let's keep it
simple. The user knows what they're trying to do, so let
them tell us.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -75,10 +75,6 @@ because a fetch would overwrite any local commits.
 +
 When a push mirror is created with `\--mirror=push`, then `git push`
 will always behave as if `\--mirror` was passed.
-+
-The option `\--mirror` (with no type) sets up both push and fetch
-mirror configuration. It is kept for historical purposes, and is
-probably not what you want.
 
 'rename'::
 

builtin/remote.c

@@ -136,13 +136,19 @@ static int add_branch(const char *key, const char *branchname,
 	return git_config_set_multivar(key, tmp->buf, "^$", 0);
 }
 
+static const char mirror_advice[] =
+"--mirror is dangerous and deprecated; please\n"
+"\t use --mirror=fetch or --mirror=push instead";
+
 static int parse_mirror_opt(const struct option *opt, const char *arg, int not)
 {
 	unsigned *mirror = opt->value;
 	if (not)
 		*mirror = MIRROR_NONE;
-	else if (!arg)
+	else if (!arg) {
+		warning("%s", mirror_advice);
 		*mirror = MIRROR_BOTH;
+	}
 	else if (!strcmp(arg, "fetch"))
 		*mirror = MIRROR_FETCH;
 	else if (!strcmp(arg, "push"))