Fix bugs in _hydro_pwd

[blt-r]

There were a couple of bugs in the implementation of _hydro_pwd which caused path to be rendered incorrectly:

• if there is a parent directory named the same way as git root, it will be "highlighted" instead of the actual git root.
  For example, if you are in /foo/repo/bar/repo/baz and the second repo is git root, the first one will be highlighted.

• if you are in directory named : (but not in git repo) it will not be shown.
  For example, if you are in /tmp/: you will see /t/ instead of /t/:.

• if one of your parent directories is : and you are in git repo, the name of the repo will be shown instead of :.
  For example, if you are in /foo/:/bar/repo/baz you will see /f/repo/b/r/baz instead of /f/:/b/repo/baz.

• if fish_prompt_pwd_dir_length is set to 0 and one of the parent directories starts with dot, the directory shown will start with dot.
  For example, if you are in /home/me/.config/fish you will see .fish instead of fish.

• if you are in a directory that looks like your home directory but isn't, it will be replaced with ~.
  For example, if you are in /tmp/home/me you will see /tmp~.

(Sorry I didn't make a commit per bug fix, probably would have been more readable that way)

[blt-r]

Also, it runs for every single prompt (or even twice per prompt?) instead of only when $PWD is changing. This is caused by --on-variable fish_prompt_pwd_dir_length. I know the standard prompt_pwd is setting the $fish_prompt_pwd_dir_length, and there is probably something else in fish that sets it.

I would recommend not dealing with $fish_prompt_pwd_dir_length and instead have something like $hydro_pwd_dir_length