Allow negative matches for commit messages in push rules again

Description

In 10.4 we switched from the ruby regular expressions to RE2 as the regex engine used for commit messages in push rules for security reasons.
This switch also made it impossible to create negative matches for commit messages afaik.
RE2 doesn't support negative look aheads.
This is somewhat a regression in functionality.

Proposal

Add a new push rule Commit message (negative match).
The code is the same as the existing push rule Commit message.
The only difference would be that we reject the commit if it matches instead of accepting it.

/cc @jamedjo