repo-hash.sh : create a short repository hash file.
usage: repo-hash.sh REPOSITORY [HASHFILE]

If HASHFILE is not given, print the hash to the console.
    Returns always returncode 0.

If HASHFILE is given:
    Create HASHFILE if it doesn't exist of if the hash has changed
    with respect to the existing file.
    Returns 0 if the hash has changed and 1 if it has not changed.

The type of the source repository is automatically recognized,
supported repositories:
    git mercurial darcs

options:
-h --help   : this help
-v --verbose: Show what the script does
-n --dry-run: Just show what the script would do