Support creating hard links (in additional to symlinks)

With for example, gitman.yml:

```
location: vendor/gitman
sources:
- repo: https://github.com/jacebrowning/gitman.git
name: gitman
rev: main
type: git
links:
- source: doc
target: vendor/doc/gitman
```

And .gitignore:
```
vendor/gitman
```

`vendor/doc/gitman` will be a symlink, and content in `vendor/gitman` will be ignored.

For my project I want to keep copies of the libraries I use, and since git does not follow symlinks the approach above does not work.

I could of course remove the entry in .gitignore, but then I get a lot of unneeded resources.

Hard links seem to be the way to resolve this. Suggested implementation, gitman.yml:

```
location: vendor/gitman
sources:
- repo: https://github.com/jacebrowning/gitman.git
name: gitman
rev: main
type: git
hardlinks:
- source: doc
target: vendor/doc/gitman
```

`vendor/doc/gitman` would be a regular directory, each file inside would be a hardlink, and any nesting directory would be recreated with the same name as a directory (not symlink). Like a copy (cp -r) but the files are hard linked instead of copies.

Of course hard links has some limitations, has to be on same file system for instance. So an option to fallback to a copy would perhaps required.

Start by reading the existing handling for the `links` entries in `gitman.yml` and the command entry point that creates symlinks. Define how the proposed `hardlinks` entries should recreate directories, handle cross-filesystem failures, and optionally fall back to copying; done means the configured files are linked as regular files while existing symlink behavior remains unchanged.

Written by the indexing model from the issue text.