Account for more detached head states

[tmwitczak]

• GitBranch provider hasn't been accepting all possible detached head characters up until now. Example: I had a detached head with a tag name 0.5.0 – current implementation would cut it and display as 0.
• When editing files during a rebase, statusline would constantly and unsuccessfully try to fetch a detached head branch name causing extreme slowdown with every movement. It wouldn't work because the naming pattern is different here: (no branch, rebasing [...]) instead of (detached at/from [...]).