Pre-commit hook fails to find the plugin script in a linked worktree

[reda-alaoui]

Fixes #390

Before, from a linked worktree:

• The pre-commit base script looked for the plugin script under $(git rev-parse --git-dir)/hooks, which is .git/worktrees/<name>/hooks. That directory holds no hooks, so the commit failed.
• install-hooks run from a worktree wrote the hooks to that same unused directory.
• The plugin script ran Maven on the absolute pom path of the checkout that installed the hooks, so it formatted that checkout instead of the worktree being committed.

Now:

• The hooks are installed in $(git rev-parse --git-common-dir)/hooks, where git runs them from, and the base script looks for the plugin script there. Reinstalling removes the former --git-dir call from existing base scripts.
• The plugin script resolves the pom and the maven wrapper against $(git rev-parse --show-toplevel), the worktree being committed. A maven installation outside the repository keeps its absolute path.
• gitBaseDir() returns the work tree, instead of the parent of the git directory (.git/worktrees in a worktree).

WorktreeTest commits from a real linked worktree, with the hooks installed from the main checkout and from the worktree.