multimail: remove redundant error handling

  $ git log --format=%s --no-walk $SHA1

can never fail unless $SHA1 is an invalid SHA-1 (which is again not
possible from preceding code).  Strip this redundant error handling.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Merge remote-tracking branch 'ram/redundant-error'

Remove some redundant error handling.