add-patch: respect diff.context configuration

add-patch: respect diff.context configuration

Various builtins that use add-patch infrastructure do not respect
the user's diff.context and diff.interHunkContext file configurations.

The user may be used to seeing their diffs with customized context size,
but not in the patches "git add -p" shows them to pick from.

Teach add-patch infrastructure to read these configuration variables and
pass their values when spawning the underlying plumbing commands as
their command line option.

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