Preserving untracked files in pull action

The pull action mirrors the FTP server files. It also deletes local
files that are not on the server.

There are two ways to have untracked files at that stage:
1. git-ftp's check for a 'dirty' repository ignores untracked files.
2. If .gitignore changes between commits, untracked files can pop up.

These files are now stashed before mirroring and restored afterwards.