Merge branch 'bc/format-patch-null-from-line'

"format-patch" has learned a new option to zero-out the commit
object name on the mbox "From " line.

* bc/format-patch-null-from-line:
format-patch: check that header line has expected format
format-patch: add an option to suppress commit hash
sha1_file.c: introduce a null_oid constant

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge