[clang-format] calculate MaxInsertOffset in the original code correctly.

[djasper]

Looks good.

lib/Format/Format.cpp

fixCppIncludeInsertions(StringRef Code, const tooling::Replacements &Replaces,
   std::set<int> Priorities = {0, INT_MAX};
   for (const auto &Category : Style.IncludeCategories)
     Priorities.insert(Category.Priority);
   int FirstIncludeOffset = -1;
   // All new headers should be inserted after this offset.
   unsigned MinInsertOffset =
       getOffsetAfterHeaderGuardsAndComments(FileName, Code, Style);
   StringRef TrimmedCode = Code.drop_front(MinInsertOffset);
+  // Max insertion offset in the original code.
   unsigned MaxInsertOffset =
+      MinInsertOffset +
       getMaxHeaderInsertionOffset(FileName, TrimmedCode, Style);
   SmallVector<StringRef, 32> Lines;
   TrimmedCode.split(Lines, '\n');
   unsigned Offset = MinInsertOffset;
   unsigned NextLineOffset;
   std::set<StringRef> ExistingIncludes;
   for (auto Line : Lines) {
     NextLineOffset = std::min(Code.size(), Offset + Line.size() + 1);

unittests/Format/CleanupTest.cpp

std::string Expected = "#include \"a.h\"\n"
                          "\n"
                          "#include \"b.h\"\n"
                          "#include \"c.h\"\n";
   tooling::Replacements Replaces =
       toReplacements({createInsertion("#include \"c.h\"")});
   EXPECT_EQ(Expected, apply(Code, Replaces));
 }
 
+TEST_F(CleanUpReplacementsTest, LongCommentsInTheBeginningOfFile) {
+  std::string Code = "// Loooooooooooooooooooooooooong comment\n"
+                     "// Loooooooooooooooooooooooooong comment\n"
+                     "// Loooooooooooooooooooooooooong comment\n"
+                     "#include <string>\n"
+                     "#include <vector>\n"
+                     "\n"
+                     "#include \"a.h\"\n"
+                     "#include \"b.h\"\n";
+  std::string Expected = "// Loooooooooooooooooooooooooong comment\n"
+                         "// Loooooooooooooooooooooooooong comment\n"
+                         "// Loooooooooooooooooooooooooong comment\n"
+                         "#include <string>\n"
+                         "#include <vector>\n"
+                         "\n"
+                         "#include \"a.h\"\n"
+                         "#include \"b.h\"\n"
+                         "#include \"third.h\"\n";
+  tooling::Replacements Replaces =
+      toReplacements({createInsertion("#include \"third.h\"")});
+  Style = format::getGoogleStyle(format::FormatStyle::LanguageKind::LK_Cpp);
+  EXPECT_EQ(Expected, apply(Code, Replaces));
+}
+
 TEST_F(CleanUpReplacementsTest, CanDeleteAfterCode) {
   std::string Code = "#include \"a.h\"\n"
                      "void f() {}\n"
                      "#include \"b.h\"\n";
   std::string Expected = "#include \"a.h\"\n"
                          "void f() {}\n";
   tooling::Replacements Replaces = toReplacements({createDeletion("\"b.h\"")});
   EXPECT_EQ(Expected, apply(Code, Replaces));
 }
 
 } // end namespace
 } // end namespace format
 } // end namespace clang