Returning the store's internal `git2::Repository` instance wrapped in
a `Mutex` makes it easy to run into deadlocks. Let's return a freshly
loaded repo instance instead.
This commit starts adding support for working with a Jujube repo's
underlyng Git repo (if there is one). It does so by adding a command
for pushing from the Git repo to a remote, so you can work with your
anonymous branches in Jujube and push to a remote Git repo without
having to switch repos and copy commit hashes.
For example, `jj git push origin main` will push to the "main" branch
on the remote called "origin". The remote name (such as "origin") is
resolved in that repo. Unlike most commands, it defaults to pushing
the working copy's parent, since it is probably a mistake to push a
working copy commit to a Git repo.
I plan to add more `jj git` subcommands later. There will probably be
at least a command (or several?) for making the Git repo's refs
available in the Jujube repo.
I had tried to generate the protobuf code at build time many months
ago, but decided against it because it slowed down the build too
much. I didn't realize there was the
"cargo:rerun-if-changed=<filename>" feature that time. Given that that
exists, it seems like an obvious win to generate the source code at
build time.
I put the generated sources in `$OUT_DIR` (where [1] says they should
be), then include them in the `protos` module by using the `include!`
macro. The biggest problem with that is that I couldn't get IntelliJ
to understand it, even after enabling the experimental features
described in [2].
[1] https://doc.rust-lang.org/cargo/reference/build-script-examples.html#code-generation
[2] https://github.com/intellij-rust/intellij-rust/issues/1908#issuecomment-592773865
