Add `ProjectRootGet`, make `ProjectRootGuess` use it

There should be a function, which does not guess, but actually returns
'', if no project root could be found.
ProjectRootGet: return early with fugitive buffers

This might get extended to match any other special buffers (netrw, etc).
See https://github.com/jmcantrell/vim-reporoot/pull/4 for some
discussion.
Refactor while loop: save some calls to fnamemodify
Fix detection of markers, which are ignored by `glob()`

`glob()` ignores &suffixes and &wildignore by default, which could be
changed (via "nosuf" arg).
I think that using `filereadable` and `isdirectory` directly is
more explicit though.
ProjectRootCD: cdcmd arg; add ProjectRootLCD command

Fixes: https://github.com/dbakker/vim-projectroot/issues/4