As the comparison uses process substitution to pass files after
conversion to DTS format, the diff header doesn't show the real
filenames, but the names of the file descriptors used:

    --- /dev/fd/63  2017-06-22 11:21:47.531637188 +0200
    +++ /dev/fd/62  2017-06-22 11:21:47.531637188 +0200

This is especially annoying when comparing a bunch of DT files in a
loop, as the output doesn't show a clue about which files it refers to.

Fix this by explicitly passing the original file names to the diff
command using the --label option, giving e.g.:

    --- arch/arm/boot/dts/r8a7791-koelsch.dtb
    +++ arch/arm/boot/dts/r8a7791-porter.dtb
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>