git-gui: Allow the user to control the number of context lines in a diff.

git-gui: Allow the user to control the number of context lines in a diff.

When displaying a diff the Git default of 3 line of context may not be
enough for a user to see what has actually changed.  Consequently we
set our own program default to 5 lines of context and then allow the
user to adjust this on a per-repository and global level through our
options dialog.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>