git deps: use branch "main" if "master" does not exist

[birkenfeld]

With more projects switching to their primary branch being called "main" instead of "master", simply adding a git dependency will fail. This is a usability issue, and since I assume the Rust project supports the use of this alternate nomenclature, it should do its part to support it.

E.g. with ureq:

error: failed to get `ureq` as a dependency of package ...

Caused by:
  failed to load source for dependency `ureq`

Caused by:
  Unable to update https://github.com/algesten/ureq

Caused by:
  failed to find branch `master`

Caused by:
  cannot locate remote-tracking branch 'origin/master'; class=Reference (4); code=NotFound (-3)

[ehuss]

Thanks for the report. This is loosely tracked in issue #3517. Support as already been implemented in #8522, but this requires a change in the format of Cargo.lock, so it takes a long time to roll out the transition. Support for it will probably start phasing in sometime next year.

[birkenfeld]

Thanks, I'll close this then.