storage:vcs:git: don't accept pygit2 version 0.17.3.

storage:vcs:git: don't accept pygit2 version 0.17.3.

My Repository.revparse_single() series was merged into pygit2 with:

  Merge: 3e9daa4 0238fb7
  Author: J. David Ibáñez <jdavid.ibp@gmail.com>
  Date:   Tue Sep 25 15:10:55 2012 +0200

    Merge remote-tracking branch 'wking/revparse'

But this happened after the 0.17.3 release.  The next pygit2 release
should contain the .revparse_single() code.

The getattr() hackery works because versions of pygit2 before 0.17.3
lacked a __version__ attribute.