Use RE2 compatible regex for irrelevant-files

Zuul has deprecated the usage of negative regex[1] and
reports WARNINGs when these are used.
This patch switches the branches regex to the supported
one.

[1] https://review.opendev.org/c/zuul/zuul/+/916141

Change-Id: I3cae6a14044010ee16f328036278a0542a03a701
1 file changed