completion: list refs from remote when remote's name matches a directory

completion: list refs from remote when remote's name matches a directory

If the remote given to __git_refs() happens to match both the name of
a configured remote and the name of a directory in the current working
directory, then that directory is assumed to be a git repository, and
listing refs from that directory will be attempted.  This is wrong,
because in such a situation git commands (e.g. 'git fetch|pull|push
<remote>' whom these refs will eventually be passed to) give
precedence to the configured remote.  Therefore, __git_refs() should
list refs from the configured remote as well.

Add the helper function __git_is_configured_remote() that checks
whether its argument matches the name of a configured remote.  Use
this helper to decide how to handle the remote passed to __git_refs().

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>