reject merges on gdb release branches?

[Joel Brobecker]

We are already rejecting merge commits on branch master.

I propose we do the same for the gdb-7.7 branch, and generally
speaking on all GDB release branches, just to make sure that
people don't accidently push a merge when they meant to cherry-pick.
Thoughts?

At the moment, it appears that this can only be done by adding
a "BRANCH_NAME = true" line in the repository's config file.
I can do that.

-- 
Joel