Support previewing and merging work tree changes into the current window

[sbatten]

Given a git repo opened in VS Code on a work tree (e.g. the main work tree) with working changes in other work trees, we should make it simple to both view the diff between the 2 work trees and apply those changes directly to the current window much like a stash apply. We should also make it simple to cleanup the work trees after merging.

[0ptim]

Love to see this. Really need locally running parallel Copilot instances to work nicely within VS Code. I think work trees are the best way to implement this.

[nguyenchristy]

Different merging cases to consider:

• Edited in worktree and/or workspace in existing file (resolve merge conflicts if any)
• New file in worktree (doesn't exist in workspace)
• Delete file in worktree (exists in workspace)
• History diverged: workspace pulled/committed after worktree branched
• Renames in worktree
• Worktree has staged changes in file1 and unstaged changes in file1 -> staged changes are prioritized in migration

[nguyenchristy]

Approaches:

Viewing diffs:

• Open diff editor. (Drawback: creates confusing UI where 'revert block' can either make changes to worktree or parent repository, depending on which repository was modified)
• Open merge editor shows clearer picture of diffs but does not align with the sole goal to view diffs

Applying changes from worktree to workspace

• Overwrite the worktree files into the workspace (drawback: makes the assumption that the worktree is exactly how you want the workspace file to be and local changes on workspace is overwritten)
• Open merge editor and accept all changes from right to left (drawback: flashes merge editor, may not be the right approach for finding diffs and applying patches)
• Related to merge editor, using underlying logic of generating diff patches to apply them (drawback: does not address Git nuances, seems more related to just changes in what's shown in text editor, inaccessible in Git extension without copying merging algorithms over)
• git merge file (has shown conflict markers even when there's no conflict)
• git diff, git apply (overwrites local changes from parent repo file, does not merge cleanly)

[nguyenchristy]

We should be relying on the current Git flow of committing, pulling, and merging those changes and not making our own merging logic, similar to the GHPR extension.