[Worktree] Find a worktree again after its folder was moved (part 5) - #731

Slice 5 of #675: find a worktree again after its folder was moved, the way Tower offers to repair one,
instead of offering only to prune it.
(It builds on #727 (slice 4) and targets its branch so the diff shows this slice alone; merge #723 and #727 first.)

Changes

• While a worktree's folder is missing, Locate Worktree Folder... takes the place of Reveal Worktree in Finder, which had nothing to reveal, the same way Lock and Unlock trade places. The sidebar row and the branch label in the history list stay identical:

  Open Worktree of Branch "X"
  Copy name "X"
  ----
  Locate Worktree Folder...   (Reveal Worktree in Finder while the folder is there)
  Lock Worktree... / Unlock Worktree
  ----
  (branch items, unchanged)
  

• The missing-folder sheet (double-clicking a dimmed worktree) gains a Locate Folder... button and a sentence saying what it is for.

• Locating opens a folder panel at the worktree's old location and runs git worktree repair <folder>. It works on a locked worktree too, which stays locked.

• A wrong folder is caught: git's own error is shown for a folder that is no worktree. Two cases git lets through are checked by GitX: a folder that already belongs to a worktree is refused before git runs, and when the chosen folder is another moved worktree's (git quietly repairs that one and exits 0), GitX reports that the worktree asked about is still missing.

Screenshots

Missing Worktree - Grayed-out and "Prunable":

Missing Worktree - Can Prune:

Missing Worktree - Can Locate:

Older git

git worktree repair arrived in git 2.29. Following the plan agreed on the issue (comment) and the version check #723 adds, Locate Worktree Folder... is shown disabled before 2.29 with a tooltip naming the version needed and the one found, and the sheet offers no Locate Folder... button.

Repairing after the main repository's folder moves (git worktree repair with no path) is not part of this slice.

Test plan

• make unit-test: 289 tests, 10 of them new: the 2.28.1/2.29 boundary; repairing a moved worktree and a moved locked one against a real repository; refusing another worktree's folder, a plain folder, and another moved worktree's folder; the menu swap in both menus and on an older git; and the sheet's buttons.
• In the Debug build, on a scratch repository with a worktree whose folder was moved: the sheet's text and buttons, Locate Folder... opening the panel, choosing the moved folder (git worktree list then shows the new path and GitX's list follows), the menu action reaching the window controller, and choosing the main worktree's folder refused with its reason.