Gerrit support: assume *-source job gets triggered with full gerrit params

The *-gerrit job which triggers the *-source job should
use the "Current build parameters" option. Otherwise
the *-source job might have a GIT_COMMIT present in
its working space which doesn't match the according
patchset under review.

So always assume that we can rely on parameter
GERRIT_PATCHSET_REVISION to fetch the patchset under
review that should be used for building gerrit stuff.

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