dirstate-entry: add `modified` property
This was already done in the Rust implementation and is a useful primitive.
The C implementation had this called `merged`, but wasn't used anywhere.
It will be used in the next changeset.
#require cargo test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/../rust
Check if Cargo.lock is up-to-date. Will fail with a 101 error code if not.
$ cargo check --locked --all --quiet
However most CIs will run `cargo build` or similar before running the tests, so we need to check if it was modified
$ testrepohg diff Cargo.lock