diff -p: squelch "diff --git" header for stat-dirty paths

diff -p: squelch "diff --git" header for stat-dirty paths

The plumbing "diff" commands look at the working tree files without
refreshing the index themselves for performance reasons (the calling
script is expected to do that upfront just once, before calling one or
more of them).  In the early days of git, they showed the "diff --git"
header before they actually ask the xdiff machinery to produce patches,
and ended up showing only these headers if the real contents are the same
and the difference they noticed was only because the stat info cached in
the index did not match that of the working tree. It was too late for the
implementation to take the header that it already emitted back.

But 3e97c7c (No diff -b/-w output for all-whitespace changes, 2009-11-19)
introduced necessary logic to keep the meta-information headers in a
strbuf and delay their output until the xdiff machinery noticed actual
changes. This was primarily in order to generate patches that ignore
whitespaces. When operating under "-w" mode, we wouldn't know if the
header is needed until we actually look at the resulting patch, so it was
a sensible thing to do, but we did not realize that the same reasoning
applies to stat-dirty paths.

Later, 296c6bb (diff: fix "git show -C -C" output when renaming a binary
file, 2010-05-26) generalized this machinery and added must_show_header
toggle.  This is turned on when the header must be shown even when there
is no patch to be produced, e.g. only the mode was changed, or the path
was renamed, without changing the contents.  However, when it did so, it
still kept the special case for the "-w" mode, which meant that the
plumbing would keep showing these phantom changes.

This corrects this historical inconsistency by allowing the plumbing to
omit paths that are only stat-dirty from its output in the same way as it
handles whitespace only changes under "-w" option.

The change in the behaviour can be seen in the updated test.

Signed-off-by: Junio C Hamano <gitster@pobox.com>