release.sh: Add new option --from-tag
With Xwayland and Xorg build from different branches sharing the same
master branch, using git describe
to find the last tag may give the
wrong tag, and the resulting git short log will be wrong.
Add a new command line option '--from-tag' to pass the last known applicable tag to use for generating the short log in the announce.
Signed-off-by: Olivier Fourdan ofourdan@redhat.com