commit-msg: Don't create message with only Change-Id

If a user aborts a commit, the commit-msg hook is still called,
but with an empty file.  We need to leave the empty file alone.

Change-Id: I13766135dac267823cb08ab76f67d2000ba2d1ce
Signed-off-by: Shawn O. Pearce <sop@google.com>