Always use git metatada when fetching the latest commit information

Since we're now able to bump both refs and versions (in comments) at the
same time, `dependency.version` may not always hold a ref now. So always
rely on metadata here.

common/lib/dependabot/git_commit_checker.rb

@@ -85,13 +85,8 @@ def branch_or_ref_in_release?(version)
     def head_commit_for_current_branch
       ref = ref_or_branch || "HEAD"
 
-      if pinned?
-        return dependency.version ||
-               local_repo_git_metadata_fetcher.head_commit_for_ref(ref)
-      end
-
-      sha = local_repo_git_metadata_fetcher.head_commit_for_ref(ref)
-      return sha if sha
+      sha = head_commit_for_local_branch(ref)
+      return sha if pinned? || sha
 
       raise Dependabot::GitDependencyReferenceNotFound, dependency.name
     end

common/spec/dependabot/git_commit_checker_spec.rb

@@ -510,31 +510,32 @@
           ref: "v1.0.0"
         }
       end
-      it { is_expected.to eq(dependency.version) }
 
-      context "without a version" do
-        let(:version) { nil }
+      let(:git_header) do
+        { "content-type" => "application/x-git-upload-pack-advertisement" }
+      end
+      let(:auth_header) { "Basic eC1hY2Nlc3MtdG9rZW46dG9rZW4=" }
 
-        let(:git_header) do
-          { "content-type" => "application/x-git-upload-pack-advertisement" }
-        end
-        let(:auth_header) { "Basic eC1hY2Nlc3MtdG9rZW46dG9rZW4=" }
+      let(:git_url) do
+        "https://github.com/gocardless/business.git" \
+          "/info/refs?service=git-upload-pack"
+      end
 
-        let(:git_url) do
-          "https://github.com/gocardless/business.git" \
-            "/info/refs?service=git-upload-pack"
+      context "that can be reached just fine" do
+        before do
+          stub_request(:get, git_url).
+            with(headers: { "Authorization" => auth_header }).
+            to_return(
+              status: 200,
+              body: fixture("git", "upload_packs", "business"),
+              headers: git_header
+            )
         end
 
-        context "that can be reached just fine" do
-          before do
-            stub_request(:get, git_url).
-              with(headers: { "Authorization" => auth_header }).
-              to_return(
-                status: 200,
-                body: fixture("git", "upload_packs", "business"),
-                headers: git_header
-              )
-          end
+        it { is_expected.to eq(dependency.version) }
+
+        context "without a version" do
+          let(:version) { nil }
 
           it { is_expected.to eq("df9f605d7111b6814fe493cf8f41de3f9f0978b2") }