Most of the value of `jj fix` over a shell script is in formatting commits
other than `@`. `@::` often doesn't contain those other commits, so `-s @` is a
bad default.
We could get the same effect from `-s 'mutable() & ::@'`, but `reachable()` is
a bit more explicit and simple to read.
We could also base this on excluding `trunk()`, but that just seems like an
indirection for `mutable()` that might ignore the user's intent if they have
configured part of trunk to be mutable.
As we discovered in the `jj fix` tests,
`MergedTreeBuilder::write_tree()` doesn't try to resolve conflicts,
not even trivial ones. This patch fixes that.
