(fix) #159 by ensuring source project is used and add test

[zaquestion]

No description provided.

[codecov]

Codecov Report

Merging #174 into master will increase coverage by 0.66%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   76.32%   76.98%   +0.66%     
==========================================
  Files          38       38              
  Lines        1647     1647              
==========================================
+ Hits         1257     1268      +11     
+ Misses        270      259      -11     
  Partials      120      120

Impacted Files	Coverage Δ
cmd/mr_checkout.go	90% <100%> (+26.66%)	⬆️
internal/gitlab/gitlab.go	58.95% <0%> (+1.31%)	⬆️

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