merge: talk about the merging state after git_merge()

Since it's not necessarily obvious, mention the merging state and how to
clear it.
remote: fix tagopt test

An anonymous remote wouldn't create remote-tracking branches, so testing
we don't create them for TAGS_ALL is nonsensical. Furthermore, the name
of the supposed remote-tracking branch was also not one which would have
been created had it had a name.

Give the remote a name and test that we only create the tags when we
pass TAGS_ALL and that we do create the remote-branch branch when given
TAGS_AUTO.
Fixed GIT_REMOTE_DOWNLOAD_TAGS_ALL to behave like git