git gui: fix staging a second line to a 1-line file

git gui: fix staging a second line to a 1-line file

When a 1-line file is augmented by a second line, and the user tries to
stage that single line via the "Stage Line" context menu item, we do not
want to see "apply: corrupt patch at line 5".

The reason for this error was that the hunk header looks like this:

	@@ -1 +1,2 @@

but the existing code expects the original range always to contain a
comma. This problem is easily fixed by cutting the string "1 +1,2"
(that Git GUI formerly mistook for the starting line) at the space.

This fixes https://github.com/git-for-windows/git/issues/515

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>