Support git commit --fixup=amend:<commit> and --fixup=reword:<commit> keymaps

[KSR-Yasuda]

Besides the current keymap cf = git commit --fixup=<commit>,
support --fixup=amend:<commit> and --fixup=reword:<commit> as cfa and cfr.

Also added a keymap cff for the command of cf,
since the original keymap cf comes to wait for an extra key a or r till timeout.

[tpope]

Please follow the precedent set by Magit.

[KSR-Yasuda]

So, you mean this?

Then, the existing binding cA changes:
to be used for commit --edit --fixup=amend:<commit>;
but now commit --edit --squash=<commit>, that ought to be cn.

[tpope]

Then, the existing binding cA changes:

I guess we should have a grace period that tells users about the change. I'll go ahead and do that.