Return empty list of branches if repository is bare.

[cybe]

This pull request enables the git module to handle bare (empty) repositories when getting all branches of a repository. This failed previously, because the command to retrieve the branches git show-ref --heads exited with return code 1 which resulted in a non-nil error.

The new implementation uses git for-each-ref to iterate over all branches and outputs nothing in case the repository is bare while using a return code of 0. As the output format changed (and now is much simpler), the parsing of that output had to be changed as well.

This is in preparation of a coming pull request to gitea itself to gracefully handle bare repositories on various API operations instead of throwing HTTP-500 errors.

[ethantkoenig]

LGTM

However, we should be careful when updating gitea with these changes, because some places (e.g. RepoRef in modules/context/repo.go) assume that GetBranches() will return a non-empty slice.

[ethantkoenig]

See also go-gitea/gitea#1706

[lunny]

@ethantkoenig did you mean this will fix go-gitea/gitea#1706 ?

[ethantkoenig]

Sorry, should have been more clear. It will prevent the error from occurring, but we'll need to change some code in modules/context/repo.go to account for the fact that GetBranches() may return an empty slice.

[lunny]

LGTM, maybe need some tests.