hgext/git/gitutil.py

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

"""utilities to assist in working with pygit2"""
from __future__ import absolute_import

from mercurial.node import bin, hex, nullid

from mercurial import pycompat

pygit2_module = None


def get_pygit2():
    global pygit2_module
    if pygit2_module is None:
        try:
            import pygit2 as pygit2_module

            pygit2_module.InvalidSpecError
        except (ImportError, AttributeError):
            pass
    return pygit2_module


def togitnode(n):
    """Wrapper to convert a Mercurial binary node to a unicode hexlified node.

    pygit2 and sqlite both need nodes as strings, not bytes.
    """
    assert len(n) == 20
    return pycompat.sysstr(hex(n))


def fromgitnode(n):
    """Opposite of togitnode."""
    assert len(n) == 40
    if pycompat.ispy3:
        return bin(n.encode('ascii'))
    return bin(n)


nullgit = togitnode(nullid)