git: don't fail import when pygit2 is not install...
git: don't fail import when pygit2 is not install
`test-duplicateoptions.py` was failing on py2 for be because I didn't
have pygit2 installed. It failed because we depend on pygit2 at import
time. This patch makes it so we successfully load the git extension
even if pygit2 doesn't exist -- we just won't be able to use it in
that case.
Differential Revision:
https://phab.mercurial-scm.org/D8268