ref: Handle `git.ErrReferenceAmbiguous` in `FindBranch`

The `repo.GetReference` can throw a `git.ErrReferenceAmbiguous` error
when the provided reference is ambiguous. We shouldn't treat these
errors as internal errors as they would then be counted towards the
error budget. So catch these errors and treat them as
`NewInvalidArgument` errors.

internal/gitaly/service/ref/branches.go

@@ -26,6 +26,11 @@ func (s *server) FindBranch(ctx context.Context, req *gitalypb.FindBranchRequest
 		if errors.Is(err, git.ErrReferenceNotFound) {
 			return &gitalypb.FindBranchResponse{}, nil
 		}
+
+		if errors.Is(err, git.ErrReferenceAmbiguous) {
+			return nil, structerr.NewInvalidArgument("target reference is ambiguous: %w", err)
+		}
+
 		return nil, err
 	}
 	commit, err := repo.ReadCommit(ctx, git.Revision(branchRef.Target))

internal/gitaly/service/ref/branches_test.go

@@ -87,7 +87,8 @@ func TestFailedFindBranchRequest(t *testing.T) {
 
 	ctx := testhelper.Context(t)
 	cfg, client := setupRefService(t)
-	repo, _ := gittest.CreateRepository(t, ctx, cfg)
+	repo, repoPath := gittest.CreateRepository(t, ctx, cfg)
+	gittest.WriteCommit(t, cfg, repoPath, gittest.WithBranch("branch"), gittest.WithMessage("branch"))
 
 	testCases := []struct {
 		desc        string
@@ -106,6 +107,12 @@ func TestFailedFindBranchRequest(t *testing.T) {
 			branchName:  "",
 			expectedErr: status.Error(codes.InvalidArgument, "Branch name cannot be empty"),
 		},
+		{
+			desc:        "ambiguous branch name",
+			repo:        repo,
+			branchName:  "b*",
+			expectedErr: structerr.NewInvalidArgument(`target reference is ambiguous: reference is ambiguous: conflicts with "refs/heads/branch"`),
+		},
 	}
 
 	for _, testCase := range testCases {