rebase -i -p: add the 'topic' command

rebase -i -p: add the 'topic' command

Use the 'topic' command as a convenience shortcut to start and merge topic
branches in rebase scripts:

	topic begin side-branch at 1234567 (Hello World)
	...
	topic end side-branch Merge 'side-branch' into uggaly-uggaly

is a short form for

	bookmark merge-parent-of-side-branch
	goto 1234567 (Hello World)
	...
	bookmark side-branch
	goto merge-parent-of-side-branch
	merge parents side-branch original mark1 Merge 'side-branch' ...

For further convenience, you can drop the 'at' part in 'topic begin' when
the branch should branch off of 'onto', and you can drop the 'original ...'
part when the merge message is "Merge '<topic>'".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>