Move to Go Git

[AnomalRoil]

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Summary

It has been previously discussed in #1365 and other issues that we would prefer to not have dependencies on external binaries. Git is currently such a dependency.

We should probably move away from it by relying on https://github.com/go-git/go-git

[dominikschulz]

Warning: This is only possible once there is a green checkmark next to the word "merge" in https://github.com/go-git/go-git/blob/master/COMPATIBILITY.md#supported-capabilities

[dominikschulz]

Also we need to ensure that we'd keep supporting gitremote-helpers.

[AnomalRoil]

That's effectively a big blocker then and I couldn't find an open issue about it.

https://github.com/src-d/go-git/issues/745 used to discuss it before go-git was moved to its new repo, but it seems it wasn't moved to the new repo yet (https://github.com/go-git/go-git/issues/9 is still open)

[dominikschulz]

Just checking, go-git still doesn't have merge support. Seems to be either tricky and or boring so nobody wants to fix it :(

[dominikschulz]

As long as go-git doesn't have merge support this is not an option. Closing this for the time being since there has been zero movement on that feature since I first looked at it.