Merge branch 'jc/maint-diff-patch-header' into maint

"git diff-index" and its friends at the plumbing level showed the
"diff --git" header and nothing else for a path whose cached stat
info is dirty without actual difference when asked to produce a
patch. This was a longstanding bug that we could have fixed long
time ago.

By Junio C Hamano
* jc/maint-diff-patch-header:
  diff -p: squelch "diff --git" header for stat-dirty paths
  t4011: illustrate "diff-index -p" on stat-dirty paths
  t4011: modernise style