Frescobaldi now needs git installed

[Nick Payne]

The install instructions at http://frescobaldi.org/download.html need to 
add git as a dependency. Without git installed, when trying to run 
Frescobaldi I get:

address@hidden:~$ /home/nick/source/frescobaldi/frescobaldi
Traceback (most recent call last):
  File "/home/nick/source/frescobaldi/frescobaldi", line 3, in <module>
    import frescobaldi_app.main
  File "/home/nick/source/frescobaldi/frescobaldi_app/main.py", line 
176, in <module>
    main()
  File "/home/nick/source/frescobaldi/frescobaldi_app/main.py", line 
135, in main
    import mainwindow       # contains MainWindow class
  File "/home/nick/source/frescobaldi/frescobaldi_app/mainwindow.py", 
line 40, in <module>
    import menu
  File "/home/nick/source/frescobaldi/frescobaldi_app/menu.py", line 
47, in <module>
    import vcs
  File "/home/nick/source/frescobaldi/frescobaldi_app/vcs/__init__.py", 
line 46, in <module>
    app_repo = apprepo.AppRepo()
  File "/home/nick/source/frescobaldi/frescobaldi_app/vcs/apprepo.py", 
line 42, in __init__
    self._activeBranch = self.current_branch()
  File "/home/nick/source/frescobaldi/frescobaldi_app/vcs/gitrepo.py", 
line 151, in current_branch
    for branch in self.branches():
  File "/home/nick/source/frescobaldi/frescobaldi_app/vcs/gitrepo.py", 
line 137, in branches
    return [line.strip() for line in self._run_git_command('branch', args)]
  File "/home/nick/source/frescobaldi/frescobaldi_app/vcs/gitrepo.py", 
line 119, in _run_git_command
    raise GitError(str(error))
vcs.gitrepo.GitError: /bin/sh: 1: git: not found