Make GerritStatusPush also work with GitPoller source

[muks]

It would previously work with GerritChangeSource and
GerritEventLogPoller only. But using events from these Gerrit specific
changesources can be problematic for some users, and so we support
GitPoller as well.

Reviews are sent back to the Gerrit server for builds with refspecs
matching: refs/changes/n/n/n

Contributor Checklist:

• I have updated the unit tests
• I have created a file in the master/buildbot/newsfragments directory (and read the README.txt in that directory)
• I have updated the appropriate documentation

[codecov]

Codecov Report

Merging #5956 (7eb345f) into master (9e07314) will decrease coverage by 0.01%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master    #5956      +/-   ##
==========================================
- Coverage   91.81%   91.79%   -0.02%     
==========================================
  Files         340      340              
  Lines       36343    36352       +9     
==========================================
+ Hits        33367    33368       +1     
- Misses       2976     2984       +8     

Impacted Files	Coverage Δ
master/buildbot/reporters/gerrit.py	84.77% <11.11%> (-2.37%)	⬇️

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[p12tic]

Looks good in principle. This will need a unit test and a news fragment.