[1193] Fix merge strategy for gh apps

[nishkrishnan]

Fixes #1193 by introducing a small refactor to not pass duplicate Repo objects by referencing the base repository through the PullRequest object only.

This also fixes the clone url sanitizing for gh apps which wasn't accurate.

[codecov]

Codecov Report

Merging #1225 into master will increase coverage by 0.00%.
The diff coverage is 77.27%.

@@           Coverage Diff           @@
##           master    #1225   +/-   ##
=======================================
  Coverage   69.93%   69.93%           
=======================================
  Files          71       71           
  Lines        5358     5359    +1     
=======================================
+ Hits         3747     3748    +1     
  Misses       1263     1263           
  Partials      348      348           

Impacted Files	Coverage Δ
server/events/vcs/github_credentials.go	73.33% <ø> (ø)
server/events/command_runner.go	50.71% <23.07%> (-0.35%)	⬇️
server/events/github_app_working_dir.go	62.50% <100.00%> (+8.65%)	⬆️
server/events/project_command_builder.go	78.92% <100.00%> (ø)
server/events/project_command_runner.go	73.33% <100.00%> (ø)
server/events/working_dir.go	75.26% <100.00%> (ø)

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[lkysow]

Thanks Nish for contributing this back! Lyft's fork looks like ya'll have some cool features over there!