Use lua for git_cached if lua is available

Sometimes I work with a git repository which has about 20000 files, using git_cached.
If a git repository has many files, it takes much time to open git_cached unite buffer.

Because I use git_cached many times, I want git_cached to be more faster.
So I rewrite git_cached's gather_candidates using lua for vim built with lua support.

## source.gather_candidates improvement
with 20000 files (same as unite.redraw_hold_candidates with lua)

### before (use gather_vim)
about 0.40774s (0.181152s for gather_vim)

### after (use gather_lua)
about 0.26092s (0.049877s for gather_lua)