Prevent git-restore-mtime-bare.py from setting timestamps on symlinks

[sbesson]

• Use os.utime() follow_symlinks keyword available on Python 3
• Also decode the output of the subprocess as a string on Python 3

The usage of follow_symlinks is primarily motivated by the fact that apart from the top-level idr0015§ symlinks, all links are pointing to read-only data on NFS and this script results inPermission Denied`.

[joshmoore]

@manics: is this going to hold on other platforms? I seem to remember running into issues with this based on system-python or virtualenv-python etc. Maybe it's been cleaned up in the mean time.

[manics]

I don't know, so far I've never had problems when using python3

[joshmoore]

I see multiple sentences in https://www.python.org/dev/peps/pep-0394/#for-python-script-publishers that I find conflicting.