Teach diff machinery to display other prefixes than "a/" and "b/"

Teach diff machinery to display other prefixes than "a/" and "b/"

With the new options "--src-prefix=<prefix>", "--dst-prefix=<prefix>"
and "--no-prefix", you can now control the path prefixes of the diff
machinery.  These used to by hardwired to "a/" for the source prefix
and "b/" for the destination prefix.

Initial patch by Pascal Obry.  Sane option names suggested by Linus.

Signed-off-by: Johannes Schindelin <redacted>
Acked-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>