fix: Don't assume master when checking git packages for upgrades

[savetheclocktower]

Fixes #55.

It took me ages to figure out how to replicate the test-git-repo.git fixture, but i managed to create one from scratch that uses main instead of master. All existing tests pass.

[confused-Techie]

The actual code changes here are simple and seem like a no brainer.

Literally three line changes from hardcoded values to dynamic ones that will continue to work in the future as far as we can reasonably guess.

But amazingly this PR also adds tests, new spec files, and everything needed to make sure this code works flawlessly.

I'd honestly say this is a 10/10 PR, and fully say we should get it merged! Thanks a ton for the contribution!

[DeeDeeG]

LGTM!

I think there may be places elsewhere in ppm that expect master in a hard-coded way, but I think we all want to move away from that, and this is as good a start on that as any.

[ EDIT: Big thanks for adding tests, by the way! I read every line, and glanced over every added fixture file (new basic git repo), LGTM. Change to the src/ file also looks good to me, I checked that perf is the same as before (thought for a second this added a new Git.open(), but I see now it only moves that from a later part to an earlier part of the function. Excellent! ]

Thanks for the contribution!

[savetheclocktower]

I think there may be places elsewhere in ppm that expect master in a hard-coded way, but I think we all want to move away from that, and this is as good a start on that as any.

The good news there is that I didn't find any others. The only implicit assumption of master that remains is in the code that converts a TextMate bundle on GitHub to an Atom package, and I can't imagine anyone is writing new TextMate bundles these days that would have a main default branch.