Try main branch if master is not found

[countvajhula]

There are occasionally issues related to the use of main on GitHub instead of the older master convention. For example, the racquel package currently shows this error:

Checksum error An error occurred while updating the package checksum.

git: could not find requested reference
  reference: master
  repo: brown131/racquel
  context...:
   /home/pkgserver/racket/collects/net/git-checkout.rkt:403:0: select-commits
   /home/pkgserver/racket/collects/net/git-checkout.rkt:73:8
   /home/pkgserver/racket/collects/net/git-checkout.rkt:54:2: retry-loop
   /home/pkgserver/pkg-index/official/update.rkt:31:2
   /home/pkgserver/racket/collects/racket/private/list.rkt:255:2: filter
   /home/pkgserver/pkg-index/official/update.rkt:163:0: do-update!
   /home/pkgserver/pkg-index/official/common.rkt:150:0: run!
   /home/pkgserver/pkg-index/official/update.rkt:174:0: run-update!

At the moment, not explicitly indicating a branch name is treated as indicating master. Instead, it could try some standard names in order, stopping when one is found. For now this could include just main and master, and if it emerges that other services use a different main branch name, those could be added to this list.

Of course, an explicitly indicated branch name would override this default mechanism.

[jryans]

After a bit of investigation, the root cause seems to be that Racket's package handling only follows default branches for Git package sources, but not for GitHub package sources (github://, git://github.com) such as the one racquel is using.

I have proposed a fix in racket/racket#4190. Assuming that's accepted, we would then need to update the package server to use a new Racket version after the next release.

[jryans]

The fix has been merged into Racket, so the next step is waiting for the Racket 8.5 release in a few weeks or so.