Permit installing to core.hooksPath if within repo

### Summary

There's [several
threads](https://github.com/pre-commit/pre-commit/issues?q=is%3Aissue+core+hooksPath+is%3Aclosed)
around using pre-commit with global git hooks directories, but I didn't
see anything around setting `core.hooksPath` to a directory within the
repo, as the `husky` tool does.

This patch adds support for installing/uninstalling pre-commit when
`core.hooksPath` references a path within the git repo.

 ### Testing

- add a couple of simple tests
- a repo with core.hooksPath set within the repo, install/uninstall does
  the right thing
- a repo with core.hooksPath set to `../hooks`, install/uninstall fails
- a repo with core.hooksPath unset, install/uninstall works as before

pre_commit/git.py

@@ -173,9 +173,13 @@ def has_diff(*args: str, repo: str = '.') -> bool:
     return cmd_output_b(*cmd, cwd=repo, retcode=None)[0] == 1
 
 
-def has_core_hookpaths_set() -> bool:
+def get_core_hookspath() -> str:
     _, out, _ = cmd_output_b('git', 'config', 'core.hooksPath', retcode=None)
-    return bool(out.strip())
+    return out.strip().decode()
+
+
+def has_core_hookpaths_set() -> bool:
+    return bool(get_core_hookspath())
 
 
 def init_repo(path: str, remote: str) -> None: