Add git rebase

[SuperBo]

Add git rebase view, includes following changes:

• Libgit2: add rebase C functions.
• Git2: Add rebase commit ammend to extend git rebase inmemory, Repository:update_head_for_commit
• A Fugit2 Git rebase view class to rebase interactively.
• Extend libgit2 to support inmemory: commit, amend, reword, skip, resolve conflict all now can be done inmemory.

TODO: Improve Git Diff UI

[SuperBo]

It's ok to merge it now 👯‍♂️