builtin/clone: skip reading HEAD when retrieving remote

After we have set up the remote configuration in git-clone(1) we'll call
`remote_get()` to read the remote from the on-disk configuration. But
next to reading the on-disk configuration, `remote_get()` will also
cause us to try and read the repository's HEAD reference so that we can
figure out the current branch. Besides being pointless in git-clone(1)
because we're operating in an empty repository anyway, this will also
break once we move creation of the reference database to a later point
in time.

Refactor the code to introduce a new `remote_get_early()` function that
will skip reading the HEAD reference to address this issue.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.h

@@ -118,6 +118,7 @@ struct remote {
  * and configuration.
  */
 struct remote *remote_get(const char *name);
+struct remote *remote_get_early(const char *name);
 
 struct remote *pushremote_get(const char *name);
 int remote_is_configured(struct remote *remote, int in_repo);