Plugin: Determin git repository path for command execution

Capsule subprocess.check_output call into own function to make handling
of Os speciic STARTUPINFO easier.

Rename determine_working_dir to determine_git_dir, this command trys to
find the root folder of the current git repository, depending on the
open view/folder.
Plugin: Open conflict files via full path
Plugin: Reload settings file when executing any command
Plugin: Check if git executable is available and show error dialog if not
Plugin: Use os.path.dirname rather than own implementation
Docu: Add message file for version 1.4.6
Plugin: Improve Show Conflict Files functionality

- Reload file settings on every command
- Check if the git executable lies in the path, if not show an error
  dialog
- Determine the root path of the git repository, this enables us to
  work with the correct relative paths of the conflict files
   > The plugin tries to determine the path of the git repository via
     the currently active view
   > If no view is active, it uses the first open folder to determine
     the root
   > If no folder is open, the plugin gives up (why did you even
     bothered executing the command?)

Fixes #8