Add Merge Command

[marstr]

Fixes #35

[Copilot]

Pull Request Overview

This PR adds a merge command to handle multi-parent commits in the version control system. The implementation allows users to merge multiple refspecs (branches/commits) and manages merge state persistence across operations.

• Introduces a new merge command that can merge multiple refspecs into the current head
• Adds merge state tracking with JSON serialization to handle interrupted merge operations
• Integrates merge functionality with the existing commit command to support multi-parent commits

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cmd/merge.go	New file implementing the merge command with state persistence and refspec handling
cmd/commit.go	Modified to detect pending merges and pass additional parents to commit operations

Comments suppressed due to low confidence (1)

[Copilot]

The ParentNames slice is being appended to even when a merge is already in progress. This will duplicate the current head and other refspecs on subsequent merge command invocations. The append should only happen when not in progress, or the slice should be reset first.

Suggested change

	if inProg {
	err = MergeUnstowProgress(ctx, repoLoc, &mergeParams)
	if err != nil {
	logrus.Fatal("couldn't read the currently in-progress merge because: ", err)
	}
	}
	mergeParams.ParentNames = append(mergeParams.ParentNames, heads...)
	for _, head := range heads {
	if inProg {
	err = MergeUnstowProgress(ctx, repoLoc, &mergeParams)
	if err != nil {
	logrus.Fatal("couldn't read the currently in-progress merge because: ", err)
	}
	mergeParams.ParentNames = nil // Reset ParentNames to prevent duplication
	}
	mergeParams.ParentNames = append(mergeParams.ParentNames, heads...)
	for _, head := range heads {