git-gui: Fix "Stage/Unstage Line" with one line of context.

git-gui: Fix "Stage/Unstage Line" with one line of context.

To "Stage/Unstage Line" we construct a patch that contains exactly one
change (either addition or removal); the hunk header was forged by counting
the old side and adjusting the count by +/-1 for the new side. But when we
counted the context we never counted the changed line itself. If the hunk
had only one removal line and one line of context, like this:

    @@ -1,3 +1,2 @@
     context 1
    -removal
     context 2

We had constructed this patch:

    @@ -1,2 +1,1 @@
     context 1
    -removal
     context 2

which does not apply because git apply deduces that it must apply at the
end of the file. ("context 2" is considered garbage and ignored.) The fix
is that removal lines must be counted towards the context of the old side.

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