Sphinx-last-updated-by-git

* Use ``--no-show-signature`` to avoid error when ``log.showSignature`` is on
* Properly stop ``git log`` subprocess even on error (to avoid warnings)

* Handle "added" but not yet "committed" files

* Refactor to make ``git`` calls per directory instead of per file,
which makes this extension usable on big repositories
* Raise warnings instead of errors when ``git`` is not available
and when source files are not in a Git repo
* Add warning subtypes ``git.command_not_found`` and ``git.subprocess_error``
* Drop support for Python 3.5

* ``srcdir`` can now be different from ``confdir``

* Add timestamp as a ``<meta>`` tag

* Add option ``git_last_updated_timezone``