It's ok to probe every time for git-branch remove because that's
run quite rarely. For git-checkattr, it's run only once, when
starting the --batch mode, and so again the overhead is pretty minimal.
This leaves 2 places where the build version is still used.
git merge might be interactive or fail if one skews, and --no-gpg-sign
might not be pased, or might be passed to a git that doesn't understand it
if the other skews. It seems a little expensive to check the git version
each time these are used.
This doesn't seem likely to cause many problems, at least compared with
check-attr hanging on skew.
This allows using Git.Remote w/o needing to have Git.BuildVersion, which
requires configure. It will simplify github-backup when these libraries are
used there.
