git-gui: "Stage Line": Treat independent changes in adjacent lines better

git-gui: "Stage Line": Treat independent changes in adjacent lines better

Assume that we want to commit these states:

  Old state == HEAD    Intermediate state   New state
  --------------------------------------------------------
  context before       context before       context before
  old 1                new 1                new 1
  old 2                old 2                new 2
  context after        context after        context after

that is, want to commit two changes in this order:

  1. transform "old 1" into "new 1"
  2. transform "old 2" into "new 2"

[This discussion and this patch is about this very case and one other case
as outlined below; any other intermediate states that one could imagine are
not affected by this patch.]

Now assume further, that we have not staged and commited anything, but we
have already changed the working file to the new state. Then we will see
this hunk in the "Unstaged Changes":

  @@ -1,4 +1,4 @@
   context before
  -old 1
  -old 2
  +new 1
  +new 2
   context after

The obvious way to stage the intermediate state is to apply "Stage This
Line" to "-old 1" and "+new 1". Unfortunately, this resulted in this
intermediate state:

  context before
  old 2
  new 1
  context after

which is not what we wanted. In fact, it was impossible to stage the
intermediate state using "Stage Line". The crux was that if a "+" line was
staged, then the "-" lines were converted to context lines and arranged
*before* the "+" line in the forged hunk that we fed to 'git apply'.

With this patch we now treat "+" lines that are staged differently. In
particular, the "-" lines before the "+" block are moved *after* the
staged "+" line. Now it is possible to get the correct intermediate state
by staging "-old 1" and "+new 1". Problem solved.

But there is a catch.

Noticing that we didn't get the right intermediate state by staging
"-old 1" and "+new 1", we could have had the idea to stage the complete
hunk and to *unstage* "-old 2" and "+new 2". But... the result is the same.
The reason is that there is the exact symmetric problem with unstaging the
last "-" and "+" line that are in adjacent blocks of "-" and "+" lines.

This patch does *not* change the way in which "-" lines are *unstaged*.

Why? Because if we did (i.e. move "+" lines before the "-" line after
converting them to context lines), then it would be impossible to stage
this intermediate state:

  context before
  old 1
  new 2
  context after

that is, it would be impossible to stage the two independet changes in the
opposite order.

Let's look at this case a bit further: The obvious way to get this
intermediate state would be to apply "Stage This Line" to "-old 2" and
"+new 2". Before this patch, this worked as expected. With this patch, it
does not work as expected, but it can still be achieved by first staging
the entire hunk, then *unstaging* "-old 1" and "+new 1".

In summary, this patch makes a common case possible, at the expense that
a less common case is made more complicated for the user.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>