CTest: Parse empty Git commits correctly

Git's diff-tree format has no '\n'-terminated blank line at the end of
its commit message body block if there are no diff lines.  Instead the
message body is terminated by '\0' and there is no diff section.  Teach
CTest to parse the format in this case.

Source/CTest/cmCTestGIT.cxx

@@ -317,8 +317,12 @@ protected:
    \n
        Log message indented by (4) spaces\n
        (even blank lines have the spaces)\n
+ [[
    \n
    [Diff format]
+ OR
+   \0
+ ]]
 
    The header may have more fields.  See 'git help diff-tree'.
 */
@@ -372,6 +376,11 @@ private:
     {
     if(this->Line.empty())
       {
+      if(this->Section == SectionBody && this->LineEnd == '\0')
+        {
+        // Skip SectionDiff
+        this->NextSection();
+        }
       this->NextSection();
       }
     else

Source/cmProcessTools.cxx

@@ -44,7 +44,7 @@ void cmProcessTools::RunProcess(struct cmsysProcess_s* cp,
 
 //----------------------------------------------------------------------------
 cmProcessTools::LineParser::LineParser(char sep, bool ignoreCR):
-  Separator(sep), IgnoreCR(ignoreCR), Log(0), Prefix(0)
+  Separator(sep), IgnoreCR(ignoreCR), Log(0), Prefix(0), LineEnd('\0')
 {
 }
 
@@ -61,8 +61,10 @@ bool cmProcessTools::LineParser::ProcessChunk(const char* first, int length)
   const char* last = first + length;
   for(const char* c = first; c != last; ++c)
     {
-    if(*c == this->Separator)
+    if(*c == this->Separator || *c == '\0')
       {
+      this->LineEnd = *c;
+
       // Log this line.
       if(this->Log && this->Prefix)
         {

Source/cmProcessTools.h

@@ -55,6 +55,7 @@ public:
     bool IgnoreCR;
     std::ostream* Log;
     const char* Prefix;
+    char LineEnd;
     std::string Line;
     virtual bool ProcessChunk(const char* data, int length);
 

Tests/CTestUpdateGIT.cmake.in

@@ -131,6 +131,22 @@ run_child(
   COMMAND ${GIT} submodule update
   )
 
+# Save the first revision name.
+execute_process(
+  WORKING_DIRECTORY ${TOP}/user-source
+  COMMAND ${GIT} rev-parse HEAD
+  OUTPUT_VARIABLE revision1
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+
+#-----------------------------------------------------------------------------
+# Create an empty commit.
+message("Creating empty commit...")
+run_child(
+  WORKING_DIRECTORY ${TOP}/user-source
+  COMMAND ${GIT} commit --allow-empty -m "Empty commit"
+  )
+
 #-----------------------------------------------------------------------------
 # Make changes in the working tree.
 message("Changing content...")
@@ -196,7 +212,7 @@ macro(rewind_source src_dir)
   message("Backing up to revision 1...")
   run_child(
     WORKING_DIRECTORY ${TOP}/${src_dir}
-    COMMAND ${GIT} reset --hard origin/master~2
+    COMMAND ${GIT} reset --hard ${revision1}
     )
   run_child(
     WORKING_DIRECTORY ${TOP}/${src_dir}
@@ -261,7 +277,7 @@ execute_process(
   )
 execute_process(
   WORKING_DIRECTORY \"${TOP}/dash-source\"
-  COMMAND \"${GIT}\" reset --hard master~2
+  COMMAND \"${GIT}\" reset --hard ${revision1}
   )
 execute_process(
   WORKING_DIRECTORY \"${TOP}/dash-source\"