fetch: add configuration for set_head behaviour

In the current implementation, if refs/remotes/$remote/HEAD does not
exist, running fetch will create it, but if it does exist it will not do
anything, which is a somewhat safe and minimal approach. Unfortunately,
for users who wish to NOT have refs/remotes/$remote/HEAD set for any
reason (e.g. so that `git rev-parse origin` doesn't accidentally point
them somewhere they do not want to), there is no way to remove this
behaviour. On the other side of the spectrum, users may want fetch to
automatically update HEAD or at least give them a warning if something
changed on the remote.

Introduce a new setting, remote.$remote.followRemoteHEAD with four
options:

    - "never": do not ever do anything, not even create
    - "create": the current behaviour, now the default behaviour
    - "warn": print a message if remote and local HEAD is different
    - "always": silently update HEAD on every change

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.h

@@ -59,6 +59,13 @@ struct remote_state {
 void remote_state_clear(struct remote_state *remote_state);
 struct remote_state *remote_state_new(void);
 
+	enum follow_remote_head_settings {
+		FOLLOW_REMOTE_NEVER = -1,
+		FOLLOW_REMOTE_CREATE = 0,
+		FOLLOW_REMOTE_WARN = 1,
+		FOLLOW_REMOTE_ALWAYS = 2,
+	};
+
 struct remote {
 	struct hashmap_entry ent;
 
@@ -107,6 +114,8 @@ struct remote {
 	char *http_proxy_authmethod;
 
 	struct string_list server_options;
+
+	enum follow_remote_head_settings follow_remote_head;
 };
 
 /**