fcm merge --auto-log: empty lines on merging auto-logged branch

[benfitzpatrick]

If the trunk is merged into a branch with fcm merge --auto-log and then later the branch is merged
back, again using --auto-log, empty entries will appear where the trunk merge revision autolog entries should be.

For example, with a branch with this log message:

------------------------------------------------------------------------
r5 | ben | 2016-07-26 09:22:28 +0100 (Tue, 26 Jul 2016) | 6 lines

Merged into /branches/dev/ben/r1_branchy: /trunk@3 cf. /trunk@1
----
r3:
> Add foo
----

a later merge of this branch back to the trunk can give:

------------------------------------------------------------------------
r7 | bfitz | 2016-07-26 09:28:57 +0100 (Tue, 26 Jul 2016) | 14 lines

Merge branch back
Merged into /trunk: /branches/dev/bfitz/r1_branchy@6 cf. /trunk@3
----
r6:
> Add bar
----
r5:
> ----
> ----
----
r4:
> Make new branch
----

------------------------------------------------------------------------

Where the auto-log message is long, this can give a lot of otherwise-empty 4-dash lines.

Noticed by @r-sharp.