evolve: when relocating, optionally first try to do it using in-memory merge This patch adds a config option to let run evolve's relocation step using in-memory merge. It is disabled by default. When the option is on, the relocation is first attempted in memory. If that fails because of merge conflicts, it retries that commit in the working copy. There are a few reasons that I made it configurable. The most important one is that the precommit hook won't trigger when using in-memory merge. Another reason is that it lets us roll out the feature slowly to our users at Google. For now, we also update the working copy after creating the commit (in the successful case, when there are no merge conflicts). The next patch will make it so we don't do that update. Because of the unnecessary working-copy update, this patch doesn't provide any benefit on its own. Evolving 29 commits that each change one line in the hg slows down from ~4.5s to ~4.8s when the config option is on. I've added `#testcases inmemory ondisk` to select `.t` files. Almost all differences are because of the new "hit merge conflicts" message and retrying the merge. There's also one difference in `test-stabilize-order.t` caused by the different order of working copy updates (we now update the working copy at the end).