git: enable gitrepo path style, displaying repo:/path/ instead

fixes:  #2505
closes: #2507

Signed-off-by: Christian Brabandt <cb@256bit.org>

doc/airline.txt

@@ -306,6 +306,12 @@ values):
 * Display a short path in statusline: >
   let g:airline_stl_path_style = 'short'
 >
+* Display the file path relative to the git repository toplevel directory: >
+  let g:airline_stl_path_style = 'gitrepo'
+<
+  This shows the path as `reponame:path/to/file` instead of the full path.
+  Requires fugitive plugin.  Falls back to the full path for non-git files.
+
 * Display a only file name in statusline: >
   let g:airline_section_c_only_filename = 1
 >