Sort email comment groups

In changes involving C++, *.h files should appear before their
corresponding *.cc file, although their lexicographic order is the
reverse. With this change, the relative order of *.h and *.cpp file
pairs in file groups of change comment emails is corrected.

Bug: Issue 4586
Change-Id: Ic9618a07167bde570bdfeb0ea54721c763034863

gerrit-common/src/test/java/com/google/gerrit/common/data/FilenameComparatorTest.java

@@ -0,0 +1,65 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.common.data;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+
+public class FilenameComparatorTest {
+  private FilenameComparator comparator = FilenameComparator.INSTANCE;
+
+  @Test
+  public void basicPaths() {
+    assertThat(comparator.compare(
+        "abc/xyz/FileOne.java", "xyz/abc/FileTwo.java")).isLessThan(0);
+    assertThat(comparator.compare(
+        "abc/xyz/FileOne.java", "abc/xyz/FileOne.java")).isEqualTo(0);
+    assertThat(comparator.compare(
+        "zzz/yyy/FileOne.java", "abc/xyz/FileOne.java")).isGreaterThan(0);
+  }
+
+  @Test
+  public void specialPaths() {
+    assertThat(comparator.compare(
+        "ABC/xyz/FileOne.java", "/COMMIT_MSG")).isGreaterThan(0);
+    assertThat(comparator.compare(
+        "/COMMIT_MSG", "ABC/xyz/FileOne.java")).isLessThan(0);
+
+    assertThat(comparator.compare(
+        "ABC/xyz/FileOne.java", "/MERGE_LIST")).isGreaterThan(0);
+    assertThat(comparator.compare(
+        "/MERGE_LIST", "ABC/xyz/FileOne.java")).isLessThan(0);
+
+    assertThat(comparator.compare(
+        "/COMMIT_MSG", "/MERGE_LIST")).isLessThan(0);
+    assertThat(comparator.compare(
+        "/MERGE_LIST", "/COMMIT_MSG")).isGreaterThan(0);
+
+    assertThat(comparator.compare(
+        "/COMMIT_MSG", "/COMMIT_MSG")).isEqualTo(0);
+    assertThat(comparator.compare(
+        "/MERGE_LIST", "/MERGE_LIST")).isEqualTo(0);
+  }
+
+  @Test
+  public void cppExtensions() {
+    assertThat(comparator.compare("abc/file.h", "abc/file.cc")).isLessThan(0);
+    assertThat(comparator.compare("abc/file.c", "abc/file.hpp"))
+        .isGreaterThan(0);
+    assertThat(comparator.compare("abc..xyz.file.h", "abc.xyz.file.cc"))
+        .isLessThan(0);
+  }
+}