sync: implement reprojectcmd for the local half

Add `repo.reprojectcmd`, the checkout counterpart to `repo.fetchcmd`.
When set, `repo sync` runs this command instead of Git to materialize a
project's index and worktree at the target tree, while `repo` handles
ref updates directly via `git update-ref`.

This replaces Git's tree materialization steps: detaching HEAD,
fast-forwarding, and hard-resetting. Rebasing is not delegated, and the
command is skipped if HEAD is already at the target, if HEAD is ahead of
the target during fast-forward, or for MetaProjects.

The command runs in a subshell with project environment variables (such
as `REPO_TREV`). Before running, `repo` ensures no operation is in
progress and no staged changes exist. Worktree collision detection is
delegated to the command (preserving benign unstaged/untracked edits).
Afterward, `repo` verifies that HEAD was untouched and that the index
matches the target tree.

Like `repo.fetchcmd`, this requires `repo.uselocalgitdirs`. Nested
projects and submodules are unsupported; `repo sync` fails if the
manifest contains any while `repo.reprojectcmd` is enabled.

Verified end-to-end with repo init using local-gitdirs, repo.fetchcmd,
and repo.reprojectcmd ('git -C $REPO_PATH read-tree -m -u $REPO_TREV'):
  * Verified detached HEAD checkout and correct reflog generation across
    projects.
  * Verified benign unstaged edits and untracked files survive checkout.
  * Verified conflicting untracked files fail with exit 128 without
    clobbering worktree.
  * Verified staged changes fail upfront before reprojectcmd is executed.

Bug: 513329573
Change-Id: I964d24d22dccffc05a9b991ad69f3a7e93268c01
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/626281
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

docs/fetch-cmd.md

@@ -6,6 +6,9 @@ executed during `repo sync` to fetch objects, instead of using standard
 filesystems or lazy checkouts where fetching metadata and downloading file
 contents should be decoupled.
 
+The checkout half of a sync has a counterpart, `repo.reprojectcmd`; see
+`docs/reproject-cmd.md`.
+
 ## Configuration
 
 To use this feature, set the following in `.repo/manifests.git/config`: