merge: add instructions to the commit message when editing

Before f824628 (merge: use editor by default in interactive sessions,
2012-01-10), git-merge only started an editor if the user explicitly
asked for it with --edit.  Thus it seemed unlikely that the user would
need extra guidance.

After f824628 the _normal_ thing is to start an editor.  Give at least
an indication of why we are doing it.

The sentence about justification is one of the few things about
standard git that are not agnostic to the workflow that the user
chose.  However, f824628 was proposed by Linus specifically to
discourage users from merging unrelated upstream progress into topic
branches.  So we may as well take another step in the same direction.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>