Use Git-native gitignore handling

[kytta]

Closes #1

This PR makes some changes to how executables are discovered:

• all workspace folders get processed one after the other. For each folder:
  • if git executable is in $PATH AND if the current folder is [in] a repo:
    • use git ls-files to find all executables inside the repo
  • if current folder is not in a Git repo or there is no git
    • use os.walk and check every file for executability
    • directories named .git are always ignored

This adds some breaking changes, namely:

• if Git is not available, .gitignore is not taken into account any more
  • I believe this makes sense, as you wouldn't really work with a Git-powered repo without Git available
• the methods counting the executables are generator functions (may speed things up)

TODO:

• [x] update docs

[tonsky]

It is absolutely possible to work with git without having git installed. Some GUI clients ship with their own version of git that isn’t exposed. Git might not be on a PATH from Sublime env but on a PATH with interactive REPL.

We might want to find untracked files too (user created a file but didn’t make a commit yet)

We can use git if present (with the change for untracked files) and fall back to current impl if not found.