Add git version number to org-version information
Adds the output of 'git describe' to the org-version string if we are running
from a git repository. This identifies exactly what commit is checked out in
the org-mode git repository when reporting the org-mode version number.
org-version returns something like:
Org-mode version 6.29trans (release_6.29c.42.g5996)
which shows 6.29trans (somewhere after the last release tag) which
is 42 commits after release_6.29c at git commit 5996.
The git describe information is not included if org mode is not
running from a git repository.