lily-git: amending patches

[Graham Percival]

We either need one more button for lily-git, or one less.

Taking a completely hypthetical example, suppose a new doc
contributors writes a nice addition for the docs, but only uses one
space after a period (the heretic!).  I ask for an updated patch.  Our
fearless contributor edits the file accordingly, then presses "2.
local commit" and "3. generate patches"  (or whatever the buttons
are).

He now has two patches; one of which is just fixing the mistakes in
the previous one.


I see two possibilities:
- "one less button": eliminate the "2. local commit"; just make button
3 produce a patch including _any_ difference between the source tree
and origin.
- "one more button": add a button for "un-commit", which takes the
patch out of the git history, but leaves the files modified.  The
contributor can then edit the file and do a "local commit" like
normal.
This would require a prompt for which local commit to un-commit.  I
think we can assume that contributors can recognize the correct patch
to un-commit; they'll generally only have one or two.  And if they
*do* get confused, it'll be a good lesson to write better changelog
message.  :)

Cheers,
- Graham