Remove duplicated code in DiffFormatter

Remove duplicated code in DiffFormatter

Instead of trying to stream out the header, we can drop a redundant
code path by formatting the header into a temporary buffer and then
streaming out the actual line differences later.

Its a small amount of unnecessary work to buffer the file header,
but these are typically very tiny so the cost to format and reparse
is relatively low.

Change-Id: Id14a527a74ee0bd7e07f46fdec760c22b02d5bdf
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>