patch: tweaks

patch: tweaks

1) Eliminate spurious warnings about possibly reduced hunks.  Instead only
note a possible location if we don't find a non-reversed place to apply it.
(Otherwise, hunks that move code forward around within a file look reversed at
the removal site.)

2) Open the source file read-only, so we can apply patches to read-only files.

Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>