Add Merge method

[akamensky]

Somehow this project has fetch and pull, but missing merge?

FWIW, pull is a fetch immediately followed by merge, so how was the pull implemented in the first place?

For my case:

I am running in-memory bare repo clone. Periodically I need to fetch updates for it and check the state of the file in repository. On bare repo this should be:

1. git fetch origin master
2. git merge origin/master master
3. head -> tree -> file object -> check contents

However in the absence of merge functionality I have to:

1. Fetch
2. .Reference("refs/remotes/origin/HEAD") instead of just .Head()
3. then lookup the file

[fcharlie]

Implementing the merge feature in go-git is not an easy task; currently, there is no suitable implementation of the diff3 algorithm in Golang.

The few diff3 implementations that exist each have their own problems: nasdf/diff3 relies on github.com/sergi/go-diff just like go-git, but unfortunately, sergi/go-diff is currently erroneous. Meanwhile, epiclabs-io/diff3 is a reimplementation of node-diff3, based on the LCS algorithm; however, it is regrettable that the LGPL 3.0 licensing agreement may not be a good choice.

The maintainer of libgit2, Edward Thomson, has separated the diff3 implementation of git into the libgit2/xdiff project. This implementation has better compatibility with git and can be used with cgo or be rewritten in Golang, though this is also a significant undertaking.

In summary, implementing the merge functionality in go-git still presents some challenges; beyond the diff3 algorithm, there are many details that need to be addressed.

CC: @pjbgf

[gmax79]

Please make it possible to create merge commit without diff3 functionality. Leave the task of filling the commit to the program code. Just need to make a method that creates a new commit based on 2 parent ones and function that will be called for the merge files

[enisdenjo]

epiclabs-io/diff3 is now MIT