feat: Add fetch::Arguments::is_stateless() to aid proper use of arguments.

When arguments are used, haves are reset every round in stateless protocols, while
everything else is repeated. However, this also means that previously confirmed common
commits aren't repeated unless this is specifically implemented by the user of `Arguments`.

That caller can now easily determine if negotiations have to be compensated for.

Please note that `Arguments` explicitly doesn't implement repeating of all prior arguments, which
would also repeat a lot of *in-vain* haves.

Author: Byron

gix-protocol/src/fetch/arguments/mod.rs

@@ -78,6 +78,18 @@ impl Arguments {
     pub fn can_use_include_tag(&self) -> bool {
         self.supports_include_tag
     }
+    /// Return true if we will use a stateless mode of operation, which can be decided in conjunction with `transport_is_stateless`.
+    ///
+    /// * we are always stateless if the transport is stateless, i.e. doesn't support multiple interactions with a single connection.
+    /// * we are always stateless if the protocol version is `2`
+    /// * otherwise we may be stateful.
+    pub fn is_stateless(&self, transport_is_stateless: bool) -> bool {
+        #[cfg(any(feature = "async-client", feature = "blocking-client"))]
+        let res = transport_is_stateless || self.version == gix_transport::Protocol::V2;
+        #[cfg(not(any(feature = "async-client", feature = "blocking-client")))]
+        let res = transport_is_stateless;
+        res
+    }
 
     /// Add the given `id` pointing to a commit to the 'want' list.
     ///

gix-protocol/src/fetch/tests.rs

@@ -298,6 +298,8 @@ mod arguments {
             let mut out = Vec::new();
             let mut t = transport(&mut out, true);
             let mut arguments = arguments_v2(["feature-a", "shallow"].iter().copied());
+            assert!(arguments.is_stateless(true), "V2 is stateless…");
+            assert!(arguments.is_stateless(false), "…in all cases");
 
             arguments.deepen(1);
             arguments.deepen_relative();

gix-protocol/tests/fetch/response.rs

@@ -149,6 +149,14 @@ mod v1 {
         async fn all() -> crate::Result {
             let (caps, _) = Capabilities::from_bytes(&b"7814e8a05a59c0cf5fb186661d1551c75d1299b5 HEAD\0multi_ack thin-pack filter side-band side-band-64k ofs-delta shallow deepen-since deepen-not deepen-relative no-progress include-tag multi_ack_detailed symref=HEAD:refs/heads/master object-format=sha1 agent=git/2.28.0"[..])?;
             let mut args = fetch::Arguments::new(Protocol::V1, Command::Fetch.default_features(Protocol::V1, &caps));
+            assert!(
+                args.is_stateless(true /* transport is stateless */),
+                "V1 is stateless if the transport is connection oriented"
+            );
+            assert!(
+                !args.is_stateless(false /* transport is stateless */),
+                "otherwise V1 is stateful"
+            );
             assert!(args.can_use_shallow());
             assert!(args.can_use_deepen());
             assert!(args.can_use_deepen_not());