Allow multiple commits in a merge request

[yaoyuannnn]

As discussed in #11, there are use cases where we would want multiple commits to exist in a merge request, while still maintaining the stacked merge request hierarchy. Initial idea was having a "git lab -i" command to let the user interactively decide how to group the commits (similar to the same flag in git rebase -i). An example is shown below, in which commit 2 and commit 3 will be combined into the same MR as commit 1.

pick 1568d commit 0.
pick 6bb03 commit 1.
combine 8abc3 commit 2.
combine d4568 commit 3.
pick 89b03 commit 4.

# Review commits using GerritLab.
#
# Commands:
# p, pick <commit> = use commit for an individual MR.
# c, combine <commit> = combine commit to be in the same MR as the previous commit.

[dancysoft]

Given the hard one-commit-one-MR setup of everything, I don't think this change is going to work out.