[BUG] Allow 4 charachter SHA in /src/commit

- Adjust the `RepoRefByType` middleware to allow for commit SHAs that
are as short as 4 characters (the minium that Git requires).
- Integration test added.
- Follow up to 4d76bbeda7
- Resolves #4781

tests/integration/repo_test.go

@@ -710,6 +710,13 @@ func TestCommitView(t *testing.T) {
 		doc := NewHTMLParser(t, resp.Body)
 		commitTitle := doc.Find(".commit-summary").Text()
 		assert.Contains(t, commitTitle, "Initial commit")
+
+		req = NewRequest(t, "GET", "/user2/repo1/src/commit/65f1")
+		resp = MakeRequest(t, req, http.StatusOK)
+
+		doc = NewHTMLParser(t, resp.Body)
+		commitTitle = doc.Find(".shortsha").Text()
+		assert.Contains(t, commitTitle, "65f1bf27bc")
 	})
 
 	t.Run("Full commit ID", func(t *testing.T) {