Feature: new commit flavors fixup! and squash!

[jreybert]

Default mappings could be CF and CS (previous CF mapping could be renamed CE for commit extend)

When user choose stash or fixup commit mode, a git log is shown in quick fix window,and user can choose which commit to fixup/squash from here.

[jreybert]

CF, for commit --fixup will show up log with N last commits (N should be definable). If user choose first commit, it makes a commit --amend, otherwise it creates a fixup commit.

It should open a new window, on bottom, with fancy colors ;) in read only mode. Default log format should be --oneline, but also should be definable. Then, user can search into its commits, and when cursor is on the wanted commit, press enter: a commit is created with "fixup! SHA1 msg".

Squash seems to fit witht he exact same pattern.