fix(vcs): keep local changes and staging intact across Git operations; roll back partial multi-repository operations (#1701)

* fix(vcs): keep local changes and staging intact across commit, checkout and history edits

- Commit: changes left out of a commit keep their staged state. The commit is
  made from a copy of the index (included files staged there, excluded ones
  reset to HEAD), so the user's index and other repositories are untouched.
- Smart Checkout restores staged changes as staged; untracked files a checkout
  would overwrite are listed instead of a raw git error.
- Edit Commit Message rewrites the commit and its descendants without a rebase,
  so the working tree and local changes stay as they are.
- Edit Commit Message, Drop and Squash offer Undo in their notification while
  the branch hasn't moved since.
- Interactive rebases (Drop, Squash, Interactively Rebase) put local changes
  aside themselves and bring them back with their staged state once the rebase
  is over, also after a stop for conflicts.
- Undo Commit asks for the target changelist (a new one named after the
  commit) and puts the commit message back into the commit message field.
- Cherry-pick and revert skip commits whose changes are already applied instead
  of stopping with an empty commit; commits cherry-picked from a protected
  branch get "(cherry picked from commit …)".
- Commit tool window: Commit File…, Accept Yours / Accept Theirs on conflicted
  files and Copy as Patch to Clipboard.

* fix(vcs): restore staged changes after merge, rebase and update; roll back partial multi-repository operations

- Merge, rebase, pull / Update Project and the rebase of a diverged checkout put
  local changes aside themselves and bring them back with --index, so staged
  changes stay staged (git's --autostash restored them unstaged). An operation
  stopped on conflicts keeps them aside until it is continued, skipped, aborted
  or concluded by a commit.
- Checkout, New Branch, Merge, Rebase and Delete over several repositories:
  when one repository fails after others succeeded, ask whether to roll those
  back (check out the previous branch, delete the new branch, reset to the
  commit before, restore the branch); not when it stopped on conflicts.
- Fixup… / Squash Into…: while the prepared message is in the field, the second
  commit button is Commit and Rebase, which commits and autosquashes the new
  commit into its target.
- Undo on a history rewrite replaces its notification, so the link is gone once
  used.

src/features/vcs/commitStore.ts

@@ -1,6 +1,7 @@
 /** Commit tool window state shared with actions (selection, message draft). */
 import { create } from "zustand";
 import { call, commands } from "@/platform/ipc";
+import type { Commit } from "@/bindings";
 import type { CheckState } from "./changes";
 import type { CommitProblem } from "./commitChecks";
 import { currentRepo, useVcs } from "./store";
@@ -37,6 +38,8 @@ interface CommitState {
   selectedList: string | null;
   /** Selected shelf tree key: shelf id, optionally followed by NUL and a file path. */
   selectedShelf: string | null;
+  /** Commit a Fixup… / Squash Into… message was prepared for: offers Commit and Rebase. */
+  fixupTarget: Commit | null;
 }
 
 export type CommitTab = "changes" | "shelf" | "stashes";
@@ -60,8 +63,17 @@ export const useCommit = create<CommitState>(() => ({
   tab: "changes",
   selectedList: null,
   selectedShelf: null,
+  fixupTarget: null,
 }));
 
+/** The Fixup… / Squash Into… target while the message still is the one prepared for it. */
+export function pendingFixup(message: string, target: Commit | null): Commit | null {
+  if (!target) return null;
+  return message.startsWith(`fixup! ${target.summary}`) || message.startsWith(`squash! ${target.summary}`)
+    ? target
+    : null;
+}
+
 function readBool(key: string, fallback: boolean): boolean {
   try {
     const v = localStorage.getItem(key);