Add Merge with ff-only

[john-cai]

Add a Merge function that behaves like git merge. This is a first
iteration that only supports --ff-only, which is the simplest type of
merge.

[sanderdekoning]

To others active in the repo could this be the start of implementing merge compatibility? (https://github.com/src-d/go-git/blob/master/COMPATIBILITY.md). Is this in development by anyone else? We can help (@john-cai)

[john-cai]

@sanderdekoning yes, that's the goal. I wanted to start out with the simplest version and then iterate forward. I think next would be implementing the non-ff 3-way merge

[sanderdekoning]

That's great! I'll gladly try to assist and contribute. Do note that I'm new to this repository. Who would you say can assist where necessary/provide code review @john-cai? (@mcuadros?)

[john-cai]

@sanderdekoning yes, a review sounds like a good start.

[DrewFromIQ]

Merge functionality would be very helpful for a project I'm working on at the moment. Would it be possible to get this PR completed? @mcuadros @smola

[rcontreras-te]

--ff-only merge would at least be a start. Just wondering why the PR hasn't even been considered, one way or the other, for... almost 11 months?

[john-cai]

@mcuadros could you consider reviewing this PR?

[fhochleitner]

we would also very much appreciate a way of (at least) ff merging in the library. as we are implementing something for automatic merges during workflows and hate to escape from standard go implementation to command execution

[john-cai]

@pjbgf would you mind reviewing and merging this one as it's been around for quite some time? thank you!

[pjbgf]

@john-cai I had a quick look on your PR and here's some initial feedback:

• Although this PR only supports FF, it is important to make clear what the long-term API to support all merge options would look like, so that we ensure backwards compatibility as those features are implemented. MergeOptions.FFOnly as boolean seems quite restrictive, and does not seem to match the patterns we use across similar APIs (e.g. ResetMode).
• Making IsFastForward public where it is does not feel like idiomatic Go code. I think it would probably be better placed as a method of EncodedObjectStorer.
• The compatibility.md should be updated as part of this PR.

[alita1991]

Hi, is possible to also include support for merging with -Xours and -Xtheirs?

[patgmiller]

@john-cai are you still working on this? I might be interested in contributing to merge features and prefer not to duplicate efforts.

[asahasrabuddhe]

@patgmiller @john-cai I would like to see this happen too. I am happy to pledge my time to make this happen.

[oshiro3]

Many people, including myself, are looking forward to the merge feature being implemented in go-git.
But the merge function is huge and needs to be built on a series of small goals.
Perhaps the MergeOptions structure discussed in this PR should be the first thing to be decided rather than making it smaller.
Rather than continuing the discussion in this PR, I think we should open a discussions or some other place to determine the first goal for the implementation of the merge function.

[pjbgf]

Closing in favour of #1044.