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

[Ian Clatworthy]

Vincent Ladeuil wrote:

> This is far move invasive and ugly as hell, but the best I can do
> with the given constraints.

As John suggested, if --show-ids is the solution/workaround, then
the rev-ids should be shown for TREE and MERGE-SOURCE, not just
BASE-REVISION.

Does the --show-ids option ever make sense without --show-base?
If not, I'd be tempted to call the new option --show-base-with-ids.
(Implementation-wise, maybe make it a ListOption with --show-base
so that one overrides the other if both are given.) Alternatively,
we should warn "ignoring --show-ids as --show-base option not given"
or something like that.

bb:resubmit

> +    * Add the revision id after the 'BASE-REVISION' conflict marker inserted
> +      when --show-base is used for merge.
> +      (Vincent Ladeuil, #306879)

This needs to mention --show-ids (or --show-base-with-ids).

> +                             '>>>>>>> MERGE-SOURCE\n',
> +                             tree.get_file_text(tree.path2id('hello')))
> +    def test_show_base_show_ids(self):

As John mentioned, blank line needed here.

Ian C.