[MERGE] Fix #306879 by mentioning the base revision id in the 'BASE' conflict marker lines

[Aaron Bentley]

Vincent Ladeuil wrote:
> Simple fix which add the revid of the base revision after the
> 'BASE-REVISION' marker.

I'd rather not use revision-ids where we can avoid it.

Since we can use dotted revnos, I'd rather use them.

Additionally, your fix is bogus because it will emit a revision id when
the basis tree is a working tree with uncommitted changes (ie merge
--uncommitted).

Aaron