built-in app -p: allow selecting a mode change as a "hunk"

built-in app -p: allow selecting a mode change as a "hunk"

This imitates the way the Perl version treats mode changes: it offers
the mode change up for the user to decide, as if it was a diff hunk.

In contrast to the Perl version, we make use of the fact that the mode
line is the first hunk, and explicitly strip out that line from the diff
header if that "hunk" was not selected to be applied, and skipping that
hunk while coalescing the diff. The Perl version plays some kind of diff
line lego instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>