Rename `g:gissues_default_remote` also in the doc

This was renamed during merging (e.g. in f184e4e), but not in the doc.
Handle error with getting remotes and move it up

Fixes #112: Gissues throws error when not in Git repo
Fix remote_ref command: use `--abbrev-ref`

This removes the prefix `refs/remotes/` from the name/output.
Merge pull request #113 from blueyed/fix-issue-112-etc

Fix issue 112 etc