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>