add -p (built-in): imitate `xdl_format_hunk_hdr()` generating hunk headers

add -p (built-in): imitate `xdl_format_hunk_hdr()` generating hunk headers

In libxdiff, imitating GNU diff, the hunk headers only show the line
count if it is different from 1. When splitting hunks, the Perl version
of `git add -p` already imitates this. Let's do the same in the built-in
version of said command.

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