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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.