Bundle GCM under bin

[niik]

After talking with @dscho I now know that we shouldn't be sticking arbitrary stuff into libexec/git-core, that's reserved for... ya know... git-core.

So I've moved the git credential manager into the bin/ folder as per Johannes suggestion. We still drop Git LFS into libexec/git-core and I guess we should stop doing that as well but we've had LFS in there forever so let's start with just GCM and tackle LFS in a follow-up PR.

[dscho]

Does this need any change in how git is called, i.e. augmenting the PATH so that the executables in bin will be found?

[niik]

Does this need any change in how git is called, i.e. augmenting the PATH so that the executables in bin will be found?

Came back to merge this and looked into this. TL;DR, yes, we need to amend the PATH for macOS and Linux. On Windows we already add the bin folder to the PATH: https://github.com/desktop/dugite/blob/8da03595ab67e7796d3741a556de39aa7b0f35df/lib/git-environment.ts#L91-L99. Gonna hold off on merging this until we're ready on the dugite side.