Support s-git merge

[JJLovesLife]

Basic 3-way merge algorithm.

• Find merge base, i.e., most recent common ancestor.
• Compare file tree, find modified / deleted / added files.
• Line based file diff resolve algorithm.
  Requirements:
  • Able to auto resolve unchanged lines, well most of the time.
  • Show and track area of conflict.

Reference:

• https://github.com/git/git/blob/master/merge-ort.c
• https://github.com/newren/how-merge-works