Refactor: Extract new crate simple-git (#1304)

`binstalk-downloader` contains stuff about http(s) before the
git code is moved into it and now it becomes http and git.

While git indeed uses http stuff, which is why I decided to put
it into binstalk-downloader, it is more than just downloading
since it is stateful (can be cached locally and updated)
where as http is stateless.

Also `binstalk-downloader`'s codegen time now increases
dramatically and it also creates extra dependencies for
binstalk-fetchers, delaying its execution.

The git code also don't use anything from `binstalk-downloader`
at all, it makes sense to be an independent crate.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/simple-git/src/cancellation_token.rs

@@ -0,0 +1,44 @@
+use std::sync::{
+    atomic::{AtomicBool, Ordering::Relaxed},
+    Arc,
+};
+
+use derive_destructure2::destructure;
+
+/// Token that can be used to cancel git operation.
+#[derive(Clone, Debug, Default)]
+pub struct GitCancellationToken(Arc<AtomicBool>);
+
+impl GitCancellationToken {
+    /// Create a guard that cancel the git operation on drop.
+    #[must_use = "You must assign the guard to a variable, \
+otherwise it is equivalent to `GitCancellationToken::cancel()`"]
+    pub fn cancel_on_drop(self) -> GitCancelOnDrop {
+        GitCancelOnDrop(self)
+    }
+
+    /// Cancel the git operation.
+    pub fn cancel(&self) {
+        self.0.store(true, Relaxed)
+    }
+
+    pub(super) fn get_atomic(&self) -> &AtomicBool {
+        &self.0
+    }
+}
+
+/// Guard used to cancel git operation on drop
+#[derive(Debug, destructure)]
+pub struct GitCancelOnDrop(GitCancellationToken);
+
+impl Drop for GitCancelOnDrop {
+    fn drop(&mut self) {
+        self.0.cancel()
+    }
+}
+impl GitCancelOnDrop {
+    /// Disarm the guard, return the token.
+    pub fn disarm(self) -> GitCancellationToken {
+        self.destructure().0
+    }
+}