Previous implementation of `joined` would fail this test:
```
st virtual_branches::r#virtual::tests::joined_test ... FAILED
at crates/gitbutler-core/src/virtual_branches/virtual.rs:4139:9:
assertion failed: joined(2, 3, 1, 4)
```
The `pure` functions were from a time where a `Handler` couldn't be instantiated
in full for tests, but that is not the case anymore, so there isn't any use
for the added complexity.
Previously it would watch every registered project, which could incur more work
on all parts of the application than necessary.
Now UI sends an event that indicates which project is active, allowing the
watch to be setup in that very moment. It's worth noting that the previously
watched project is automatically deregistered.
Previously, each file change both in `.git` as well as in the worktree would
cause a complete recomputation. This computation included opening a git
repository at least once (probaby more often), to make an 'is-ignored' check.
The latter is very expensive in `git2` and gets more expensive the more
files there are.
Now the repository is opened when needed, and we re-use it for all applicable
file paths.
The idea is that we don't parallelize over a channel anymore, but
instead just process filesystem events, one at a time.
This would allow each handler to become a function that gets its
state passed, and makes all the necessary calls verbatim, which
in turn makes it easy to follow what's happening.
If anything becomes to slow due to the serialization of event processing,
selective parallelization can be re-added.
Sub-Handlers are objects, and with that comes a lot of complexity.
However, there seems to be on need for it at all, and a first
steps is to turn these into methods to truly understand
what's going on.
This way, errors are perfectly associated with their respective
top-level tauri function call, which simplifies debugging.
Note that stack-traces are only shown if the `RUST_BACKTRACE`
environment variable is set.
There are no doctests and it's unlikely there will be anytime soon
as this library isn't for publishing.
The binaries did try to build unit-tests as well even though these
aren't quite feasible, so they are disabled now to reduce clutter.
