gerrit: project.config: Add [receive] section
exp/files/gerrit/project.config:
Add a [receive] section, copied from production Gerrit.
The main thing I wanted to get from this is rejectImplicitMerges = true
,
for testing T373709 (based on hashar's problem report at
https://phabricator.wikimedia.org/T366965#10095192
Bug: T373709