git gui: show diffs with a minimum of 1 context line

git gui: show diffs with a minimum of 1 context line

Staging hunks without context does not work, because line number
information would have to be recomputed for individual hunks.

Since it is already possible to stage individual lines using
'Stage Line for Commit', zero context diffs are not really
necessary for git gui.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>