Fix "Branch name doesn't conform to GIT standards" error with transformation

A hg repository I successfully cloned using git-hg-remote gave the
following error on subsequent use of "git fetch" in the
.git/fast_import_crash_XXXX log:

    fatal: Branch name doesn't conform to GIT standards: BRANCH.

I guessed that it might be the use of the "." in the branch name in hg
and applied this patch to my copy of git-hg-remote.  That seemed to fix
the problem (and allowed a whole load of other tags into the clone that
had previously been skipped during the initial 'git clone hg::'
operation).

All this patch does is add to the transformations already present in
hgref() and gitref() to replace "." in hg refnames with "__DOT__" in the
git names.  It's not ideal, but it allowed me to successfully complete
the "git fetch".

I'm not entirely convinced that the error from "git fast-import" is
valid, as I've used "." myself in tag names in pure git repos in the
past without difficulty in the past.