add experimental git worktree support

This provides initial support for using git worktrees internally
instead of our own ad-hoc symlink tree.  It's been lightly tested
which is why it's not currently exposed via --help.

When people opt-in to worktrees in an existing repo client checkout,
no projects are migrated.  Instead, only new projects will use the
worktree method.  This allows for limited testing/opting in without
having to completely blow things away or get a second checkout.

Bug: https://crbug.com/gerrit/11486
Change-Id: Ic3ff891b30940a6ba497b406b2a387e0a8517ed8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254075
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

docs/internal-fs-layout.md

@@ -102,6 +102,11 @@ support, see the [manifest-format.md] file.
     respective servers ...
 *   `subprojects/`: Like `projects/`, but for git submodules.
 *   `subproject-objects/`: Like `project-objects/`, but for git submodules.
+*   `worktrees/`: Bare checkouts of every project synced by the manifest.  The
+    filesystem layout matches the `<project name=...` setting in the manifest
+    (i.e. the path on the remote server).
+
+    This is used when git worktrees are enabled.
 
 ### Global settings
 
@@ -121,6 +126,7 @@ User controlled settings are initialized when running `repo init`.
 | repo.partialclone | `--partial-clone`         | Create [partial git clones] |
 | repo.reference    | `--reference`             | Reference repo client checkout |
 | repo.submodules   | `--submodules`            | Sync git submodules |
+| repo.worktree     | `--worktree`              | Use `git worktree` for checkouts |
 | user.email        | `--config-name`           | User's e-mail address; Copied into `.git/config` when checking out a new project |
 | user.name         | `--config-name`           | User's name; Copied into `.git/config` when checking out a new project |