merge-request.sh: Update to use new "Fixed by Commit(s)" field

Summary:
This will be used instead of the url field to track which commits need
to be merged.

This patch also drops support for version 1.x of the bugzilla CLI tool.

Reviewers: hansw, hans

Reviewed By: hans

Subscribers: hans, llvm-commits

Differential Revision: https://reviews.llvm.org/D37786

llvm-svn: 313334