ctest_update: Fix svn log and external loading

Since commit v3.7.0-rc1~132^2 (CTestSVN: Fix segfault when
CTEST_UPDATE_VERSION_ONLY is enabled, 2016-09-12) we do not properly
extract svn log or externals.  After updating we erase the information
that was loaded before updating and can no longer log the changes
between revisions to extract authors.

Fix this by only loading the repository information once, whether needed
by `NoteOldRevision`, `NoteNewRevision`, or both.

Fixes: #12630, #16646

Source/CTest/cmCTestSVN.cxx

@@ -523,8 +523,11 @@ private:
 
 void cmCTestSVN::LoadRepositories()
 {
+  if (!this->Repositories.empty()) {
+    return;
+  }
+
   // Info for root repository
-  this->Repositories.clear();
   this->Repositories.push_back(SVNInfo(""));
   this->RootInfo = &(this->Repositories.back());