build: use shallow clone for git dependencies (#17526)

Signed-off-by: xxchan <[email protected]>

.cargo/config.toml

@@ -32,3 +32,11 @@ rustflags = [
 # Flags for all targets.
 [target.'cfg(all())']
 rustflags = ["--cfg", "tokio_unstable"]
+
+# We have large git dependencies. This can make cloning faster.
+# https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#git
+# Actually we also want to prevent submodule cloning completely
+# https://github.com/rust-lang/cargo/issues/4247
+[unstable.git]
+shallow_index = true
+shallow_deps = true