Make the pre-commit hook work in a git worktree

The devcontainer only mounts the worktree directory, so git inside the
container couldn't find the gitdir that the worktree's .git file points
at. Bind-mount the main repo's git dir at the same path when we're in a
worktree.

Author: autarch

Justfile

@@ -1,13 +1,20 @@
 _dce := "devcontainer exec --workspace-folder ."
+# When we're in a git worktree, the workspace's .git is a file pointing at a
+# gitdir outside the workspace, so git doesn't work in the container unless we
+# also mount the main repo's git dir at the same path. Note that `devcontainer
+# up` reuses an existing container without comparing mounts, so a container
+# created before this mount existed needs a `just rebuild` once.
+_git_common_dir := `test -f .git && realpath "$(git rev-parse --git-common-dir)" || true`
+_git_mount := if _git_common_dir != "" { "--mount 'type=bind,source=" + _git_common_dir + ",target=" + _git_common_dir + "'" } else { "" }
 _github_token := `git config github.token 2>/dev/null || true`
 _gitlab_token := `git config gitlab.token 2>/dev/null || true`
 _codeberg_token := `git config codeberg.token 2>/dev/null || true`
 
 _up:
-    devcontainer up --workspace-folder .
+    devcontainer up --workspace-folder . {{ _git_mount }}
 
 rebuild:
-    devcontainer up --workspace-folder . --remove-existing-container
+    devcontainer up --workspace-folder . {{ _git_mount }} --remove-existing-container
 
 shell: _up
     {{ _dce }} bash -i