Instead of doing work to find the default branch just to be able to
set up the repository before doing a fetch, do a "git clone" and let
git handle it.
Use -c core.sharedrepository=0600 to get the same result as
--shared=0600 passed to init.
Use --separate-git-dir to get the git directory in $YADM_REPO. Use a
temporary dir as work tree and remove it right after the clone is
done.
When the clone is done, iterate over all missing files in $YADM_WORK
and perform a checkout. If local files exists that differ compared
with the cloned ones the local files are left intact and the user is
instructed to deal with the conflicts.
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