set upstream repository or directory

[franziskuskiefer]

./mach update should take an argument where to find the hacl-star distribution.
This can be

• a directory
• a git repository

If the upstream is a git repository

• set revision to pull

[franziskuskiefer]

There's the -s/--hacl-home argument to specify the path. This works fine but adding the option to pull from the git repository would be nice as well.