log: Fix commit headers and -p flag

We weren't flushing the commit message before the diff output, which
meant the headers and message showed randomly interleaved with the
diff rather than immediately before.

Change-Id: I6cefab8d40e9d40c937e9deb12911188fec41b26
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>