Fix for pushing to empty repo on Mononoke Git server

Summary:
In Protocol V1, the capability of the server is advertised along with the ref advertisement. This is done by null-separating the capabilities from the first ref while the rest of the refs are sent normally.

However, since the capabilities are tied to ref advertisement in the write path, having no refs mean that we won't send any capabilities to the client. This is what was wrong with the current implementation. Now the logic has been corrected to make it consistent with [the spec](https://git-scm.com/docs/pack-protocol#_reference_discovery)

Reviewed By: andreacampi

Differential Revision: D67978800

fbshipit-source-id: bb9ea76dca8f27ac3ed1f4b6b06fc55f795e4804

Author: RajivTS

eden/mononoke/git_server/src/read/capability_advertisement.rs

@@ -9,6 +9,8 @@ use core::str;
 
 use anyhow::Error;
 use bytes::Bytes;
+use gix_hash::Kind;
+use gix_hash::ObjectId;
 use gotham::mime;
 use gotham::state::FromState;
 use gotham::state::State;
@@ -84,14 +86,29 @@ async fn write_advertisement(
     .into_iter()
     .collect();
     refs.sort_by(|a, b| a.0.cmp(&b.0));
+
     let mut refs = refs.into_iter();
-    if let Some((ref_name, target)) = refs.next() {
-        let first_ref_line = ref_line(ref_name.as_str(), &target);
-        write_text_packetline(
-            format!("{}\0{}", first_ref_line, RECEIVE_PACK_CAPABILITIES).as_bytes(),
-            output,
-        )
-        .await?;
+    match refs.next() {
+        Some((ref_name, target)) => {
+            let first_ref_line = ref_line(ref_name.as_str(), &target);
+            write_text_packetline(
+                format!("{}\0{}", first_ref_line, RECEIVE_PACK_CAPABILITIES).as_bytes(),
+                output,
+            )
+            .await?;
+        }
+        None => {
+            write_text_packetline(
+                format!(
+                    "{} capabilities^{{}}\0{}",
+                    ObjectId::null(Kind::Sha1),
+                    RECEIVE_PACK_CAPABILITIES
+                )
+                .as_bytes(),
+                output,
+            )
+            .await?;
+        }
     }
     for (ref_name, target) in refs {
         write_text_packetline(ref_line(ref_name.as_str(), &target).as_bytes(), output).await?;

eden/mononoke/tests/integration/mononoke_git_server/test-mononoke-git-server-push-empty-repo.t

@@ -35,5 +35,5 @@
 
 # Push Git repository to Mononoke
   $ git_client push origin --all
-  error: failed to push some refs to 'https://*/repos/git/ro/repo.git' (glob)
-  [1]
+  To https://*/repos/git/ro/repo.git (glob)
+   * [new branch]      master_bookmark -> master_bookmark