ctest_update: Support ".git file" work trees

Commit c3781efb (Support Git upstream branch rewrites, 2010-06-08)
assumed that ".git/FETCH_HEAD" exists inside the source tree.  Fix the
implementation to handle a work tree using a ".git file" to link to its
repository.  Use "git rev-parse --git-dir" to locate the real .git dir.

Source/CTest/cmCTestGIT.cxx

@@ -84,6 +84,54 @@ void cmCTestGIT::NoteNewRevision()
              << this->NewRevision << "\n");
 }
 
+//----------------------------------------------------------------------------
+std::string cmCTestGIT::FindGitDir()
+{
+  std::string git_dir;
+
+  // Run "git rev-parse --git-dir" to locate the real .git directory.
+  const char* git = this->CommandLineTool.c_str();
+  char const* git_rev_parse[] = {git, "rev-parse", "--git-dir", 0};
+  std::string git_dir_line;
+  OneLineParser rev_parse_out(this, "rev-parse-out> ", git_dir_line);
+  OutputLogger rev_parse_err(this->Log, "rev-parse-err> ");
+  if(this->RunChild(git_rev_parse, &rev_parse_out, &rev_parse_err))
+    {
+    git_dir = git_dir_line;
+    }
+  if(git_dir.empty())
+    {
+    git_dir = ".git";
+    }
+
+  // Git reports a relative path only when the .git directory is in
+  // the current directory.
+  if(git_dir[0] == '.')
+    {
+    git_dir = this->SourceDirectory + "/" + git_dir;
+    }
+#if defined(_WIN32) && !defined(__CYGWIN__)
+  else if(git_dir[0] == '/')
+    {
+    // Cygwin Git reports a full path that Cygwin understands, but we
+    // are a Windows application.  Run "cygpath" to get Windows path.
+    std::string cygpath_exe = cmSystemTools::GetFilenamePath(git);
+    cygpath_exe += "/cygpath.exe";
+    if(cmSystemTools::FileExists(cygpath_exe.c_str()))
+      {
+      char const* cygpath[] = {cygpath_exe.c_str(), "-w", git_dir.c_str(), 0};
+      OneLineParser cygpath_out(this, "cygpath-out> ", git_dir_line);
+      OutputLogger cygpath_err(this->Log, "cygpath-err> ");
+      if(this->RunChild(cygpath, &cygpath_out, &cygpath_err))
+        {
+        git_dir = git_dir_line;
+        }
+      }
+    }
+#endif
+  return git_dir;
+}
+
 //----------------------------------------------------------------------------
 bool cmCTestGIT::UpdateByFetchAndReset()
 {
@@ -121,8 +169,13 @@ bool cmCTestGIT::UpdateByFetchAndReset()
   // Identify the merge head that would be used by "git pull".
   std::string sha1;
   {
-  std::string fetch_head = this->SourceDirectory + "/.git/FETCH_HEAD";
+  std::string fetch_head = this->FindGitDir() + "/FETCH_HEAD";
   std::ifstream fin(fetch_head.c_str(), std::ios::in | std::ios::binary);
+  if(!fin)
+    {
+    this->Log << "Unable to open " << fetch_head << "\n";
+    return false;
+    }
   std::string line;
   while(sha1.empty() && cmSystemTools::GetLineFromStream(fin, line))
     {

Source/CTest/cmCTestGIT.h

@@ -32,6 +32,8 @@ private:
   virtual void NoteNewRevision();
   virtual bool UpdateImpl();
 
+  std::string FindGitDir();
+
   bool UpdateByFetchAndReset();
   bool UpdateByCustom(std::string const& custom);
   bool UpdateInternal();

Tests/CTestUpdateGIT.cmake.in

@@ -259,6 +259,11 @@ execute_process(
   WORKING_DIRECTORY \"${TOP}\"
   COMMAND \"${GIT}\" clone \"${REPO}\" dash-source
   )
+
+# Test .git file.
+file(RENAME \"${TOP}/dash-source/.git\" \"${TOP}/dash-source/repo.git\")
+file(WRITE \"${TOP}/dash-source/.git\" \"gitdir: repo.git\n\")
+
 execute_process(
   WORKING_DIRECTORY \"${TOP}/dash-source\"
   COMMAND \"${GIT}\" reset --hard master~2