Fix submodule handling when the current branch is named after a tag

If:
1. The current branch has the same name as git tag, and
2. The current branch is set to track a remote other than `origin`, and
3. We try to update a submodule

then we'll get the following error:
```
; x c
Updating submodule src/doc/reference
remote: Total 0 (delta 0), reused 0 (delta 0), pack-reused 0
fatal: 'personal' does not appear to be a git repository
fatal: Could not read from remote repository.
```

The problem is that 1. causes `git symbolic-ref --short HEAD` to try and disambiguate the branch
from the tag using `heads/branch-name`, which breaks a previous workaround for a bug in `git
submodule update` that uses the wrong remote.

Adapt the workaround to strip `heads/` from the output.

0 files changed, 0 insertions, 0 deletions