Update create_git_tag endpoint to allow for tag hash

Summary: `remote-gitimport` makes use of `scs` to record git tags at Mononoke end. This diff updates the parameter type for the `create_git_tag` endpoint to allow for specifying the hash of the git tag being created. It is optional since the callers will be updated later to provide the tag hash and meanwhile a none value will be used.

Differential Revision: D49805555

fbshipit-source-id: 25c295183cdfdd2259add84a79518a106f210e5b

eden/mononoke/scs/if/source_control.thrift

@@ -1654,10 +1654,12 @@ struct CreateGitTagParams {
   4: optional binary pgp_signature;
   /// The changeset corresponding to the commit that was pointed at by the tag.
   5: binary target_changeset;
-  /// The identity of the service making the create git tree request.
+  /// The identity of the service making the create git tag request.
   6: optional string service_identity;
   /// The name of the tag for which the changeset is getting created
   7: string tag_name;
+  /// The git SHA1 hash of the tag
+  8: optional binary tag_hash;
 }
 
 /// Method response structures

eden/mononoke/scs_server/src/methods/git.rs

@@ -120,9 +120,23 @@ impl SourceControlServiceImpl {
             target: BonsaiAnnotatedTagTarget::Changeset(target_changeset_id),
             pgp_signature: params.pgp_signature.map(Bytes::from),
         };
+        let tag_hash = params
+            .tag_hash
+            .as_ref()
+            .map(|hash| {
+                gix_hash::oid::try_from_bytes(hash)
+                    .map(|oid| oid.to_owned())
+                    .map_err(|err| {
+                        invalid_request(format!(
+                            "Error in creating Git ObjectId from {:?}. Cause: {:#}",
+                            hash, err
+                        ))
+                    })
+            })
+            .transpose()?;
         let changeset_context = repo_ctx
             .create_annotated_tag(
-                None, // To be populated later
+                tag_hash,
                 params.tag_name,
                 params.author,
                 author_date,