add-patch: add diff.context command line overrides

add-patch: add diff.context command line overrides

This patch compliments the previous commit, where builtins that use
add-patch infrastructure now respect diff.context and
diff.interHunkContext file configurations.

In particular, this patch helps users who don't want to set persistent
context configurations or just want a way to override them on a one-time
basis, by allowing the relevant builtins to accept corresponding command
line options that override the file configurations.

This mimics commands such as diff and log, which allow for both context
file configuration and command line overrides.

Signed-off-by: Leon Michalak <leonmichalak6@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>