Add a config option for remotes to specify a foreign vcs

If this is set, the url is not required, and the transport always uses
a helper named "git-remote-<value>".

It is a separate configuration option in order to allow a sensible
configuration for foreign systems which either have no meaningful urls
for repositories or which require urls that do not specify the system
used by the repository at that location. However, this only affects
how the name of the helper is determined, not anything about the
interaction with the helper, and the contruction is such that, if the
foreign scm does happen to use a co-named url method, a url with that
method may be used directly.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1408,6 +1408,10 @@ remote.<name>.tagopt::
 	Setting this value to \--no-tags disables automatic tag following when
 	fetching from remote <name>
 
+remote.<name>.vcs::
+	Setting this to a value <vcs> will cause git to interact with
+	the remote with the git-remote-<vcs> helper.
+
 remotes.<group>::
 	The list of remotes which are fetched by "git remote update
 	<group>".  See linkgit:git-remote[1].

remote.c

@@ -54,7 +54,7 @@ static char buffer[BUF_SIZE];
 
 static int valid_remote(const struct remote *remote)
 {
-	return !!remote->url;
+	return (!!remote->url) || (!!remote->foreign_vcs);
 }
 
 static const char *alias_url(const char *url, struct rewrites *r)
@@ -444,6 +444,8 @@ static int handle_config(const char *key, const char *value, void *cb)
 	} else if (!strcmp(subkey, ".proxy")) {
 		return git_config_string((const char **)&remote->http_proxy,
 					 key, value);
+	} else if (!strcmp(subkey, ".vcs")) {
+		return git_config_string(&remote->foreign_vcs, key, value);
 	}
 	return 0;
 }

remote.h

@@ -11,6 +11,8 @@ struct remote {
 	const char *name;
 	int origin;
 
+	const char *foreign_vcs;
+
 	const char **url;
 	int url_nr;
 	int url_alloc;

transport.c

@@ -818,6 +818,11 @@ struct transport *transport_get(struct remote *remote, const char *url)
 		url = remote->url[0];
 	ret->url = url;
 
+	if (remote && remote->foreign_vcs) {
+		transport_helper_init(ret, remote->foreign_vcs);
+		return ret;
+	}
+
 	if (!prefixcmp(url, "rsync:")) {
 		ret->get_refs_list = get_refs_via_rsync;
 		ret->fetch = fetch_objs_via_rsync;