Previously the _get_avocado_git_version used current directory to
attempt to find out whether it's being executed from git sources, anyway
this fails when running from other directory, or when running
rpm-version while being inside avocado.git dir. This version uses
`os.path.dirname(__file__)`, which should be in git-tree, unless one
uses special configuration.

Note we could use `dirname(dirname(dirname(__file__)))` but that would
only work for python2 as python3 requires py2to3 fixtures, which are
usually produced in `build` directory within the sources, therefor
`git` is going to find the expected files just fine.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>