This adds support for automatically cherry-picking and testing fixes for the
release branch using 'commands' in issue comments. The two supported commands are:
/cherry-pick <commit1> <commit2> ...
Which will backport and test commits from main. And also
/branch owner/repo/branch
Which will test commits from the given branch.
Reviewed By: alexbatashev, kwk
Differential Revision: https://reviews.llvm.org/D117386
Tom Stellard