Lift constraint on current directory

Now "forge-link" works even if current directory is outside of git repository
of target file.

Add support for ssh sourcehut remotes

Simplify code to handle files not in repository root

Add support for github

Write Nix derivation for package

Correctly handle remotes other than supported forges

Fix portability to Bash

GNU Bash, unlike mksh, do not consider "cd /path/to/regular/file/.." and "cd
/path/to/regular" equivalent, and complains about regular file not being
directory instead.

Initial revision