Move default branch logic into an exported function on gps.SourceManager

[carolynvs]

PR #500 adds logic for finding the default branch of a repository to the deduceConstraint function. This should be made into a function exposed on gps.SourceMgr, and then added to the gps.SourceManager interface.

[marwan-at-work]

@carolynvs if this is a good-second-pr as well, would love to give it a shot 😄

[carolynvs]

@marwan-at-work It is! I'm going to rebase that PR soon, so that link to individual lines in the PR may stop working but just look for the deduceConstraint function.

[marwan-at-work]

@carolynvs sounds good! feel free to assign me : )

[carolynvs]

Issues can only be assigned if you're a member in the github org, but consider yourself assigned! ✨

[grepory]

Was just looking at the good-first-pr label, and it looks like this has been fixed. Closable?

[marwan-at-work]

@grepory yes i believe so!

[ibrasho]

Thanks @marwan-at-work & @grepory !