combo-layer: Stop using filterdiff

combo-layer: Stop using filterdiff

I ran into an issue where a patch just deleting a single file
within the repository (meta/recipes-devtools/m4/m4/make.patch)
would get skipped by combo-layer.

It turns out this has the patch header (commented to avoid breaking scripts):

: diff --git a/meta/recipes-devtools/m4/m4/make.patch b/meta/recipes-devtools/m4/m4/make.patch
: deleted file mode 100644
: index 79fb415..0000000
: --- a/meta/recipes-devtools/m4/m4/make.patch
: +++ /dev/null
: @@ -1,42 +0,0 @@

and this is classed as > 5 headers in filterdiff. When we piped the path
through filterdiff, the --- line disappears, then the second time we pass
through filterdiff, it shows no lines changed and the patch is assumed
to be empty and skipped.

Changing MAX_HEADERS in filterdiff is one way to fix this, another would
be to grep out "deleted file mode" lines. Instead, we can use new
git syntax to exclude files from the git format-patch instead and avoid
filterdiff entirely.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>