Update when file modified externally.
Escape shell commands on windows.
Fixed escaping
Now working with shellslash
Merge branch 'issue-84'

* issue-84:
  Now working with shellslash
  Fixed escaping
  Escape shell commands on windows.
Update signs in realtime.

Thanks to @ashb for the help with this.
Update help text.
introduce GitGutterGetHunkSummary for statusline integration.
extract updating hunk summary into another function.
add documentation for GitGutterGetHunkSummary.
Move hunk summary function to diff utility section.
Replace cd with pushd,
currently NOT working for windows
Fix calculation of removed lines for hunk summary.
Add note about Fish shell.
support using POSIX sh(1) as `set shell=/bin/sh`

Use `cd` instead of `popd` because POSIX sh(1) does not have `pushd` and
also because this plugin did not pair its `pushd` with a `popd` anyway.

Use file redirection because POSIX sh(1) does not have anonymous pipes.
Use correct line ending for realtime diff.
Fix typo.
Always use realtime diff when buffer is modified.

This was motivated by Vim regaining focus into a buffer with unsaved
changes, and the realtime diff being replaced with an out of date disk
diff.  With this commit the realtime diff will be run again.
Clear out hunks stats when file is not tracked.
Autoload.