Use different strategy to fetch git dependencies

[awvwgk]

• matches strategy of GitHub's actions/checkout workflow
• use init -> fetch -> checkout -qf FETCH_HEAD instead of current clone [-> checkout] strategy
• always works in detached HEAD
• allows to update the git dependency with the same commands (Related: github dependencies are never updated #121)
• change directory with -C command (easier to match for later Fortran implementation, available since git 1.8.5)

Can probably be implemented much nicer, but my Haskell skills don't go beyond the Try it! tutorial on haskell.org

[everythingfunctional]

I was unaware of the -C path option for git. I think this is a good idea, if for no other reason than the Fortran implementation can use exactly the same commands.

As for the implementation, I'm not sure I would have done it any different.

[milancurcic]

I don't understand the Haskell code but the PR description looks good, so 👍 from me.

[certik]

As long as tests pass and things still work, the change looks ok.