merge --no-edit: do not credit people involved in the side branch

merge --no-edit: do not credit people involved in the side branch

The credit lines "By" and "Via" to credit authors and committers for
their contributions on the side branch are meant as a hint to the
integrator to decide whom to mention in the log message text.  After
the integrator saves the message in the editor, they are meant to go
away and that is why they are commented out.

When a merge is recorded without editing the generated message,
however, its contents do not go through the normal stripspace()
and these lines are left in the merge.

Stop producing them when we know the merge is going to be recorded
without editing, i.e. when --no-edit is given.

Signed-off-by: Junio C Hamano <gitster@pobox.com>