Merges from release branches to vendor tracking branches

[Jakub Jelinek <jakub at redhat dot com>]

Hi!

Just FYI if somebody needs to do something similar, I needed to do a merge
from origin/releases/gcc-9 to our vendor branch - 
refs/vendors/redhat/heads/gcc-9-branch
This branch has some extra commits origin/releases/gcc-9 branch doesn't
have, so it is not a fast forward merge and we have the requirement that
From-SVN: shouldn't appear in commit logs of new commits.
What I did in the end that worked (thanks to Jonathan's help):
git merge r9-XXXX-gYYYYYYYYYYYY
git rebase -i # Reworded commit logs to not have From-SVN:, dealt with
              # git-merge-changelog not doing a good job for multiple
	      # entries under the same ChangeLog header
git push
git merge -s ours r9-XXXX-gYYYYYYYYYYYY # So that future git merge
					# will work
git push
# and repeat in < 100 commit chunks until done.
The ugly thing is that almost all the commit mails went out twice to
gcc-cvs, but once we are out of the From-SVN: period, one will be able to
use git merge ...; git push only.

	Jakub