source_group: Fix regression in relative FILES

Fix the check added in commit 8d93815 (source_group command ensures
that FILES arguments are actually files, 2019-04-25, v3.15.0-rc1~195^2)
to convert to an absolute path before checking for existence.

Also simplify the conversion to an absolute path.

Fixes: #19454

Author: bradking

Source/cmSourceGroupCommand.cxx

@@ -63,15 +63,6 @@ bool rootIsPrefix(const std::string& root,
   return true;
 }
 
-std::string prepareFilePathForTree(const std::string& path,
-                                   const std::string& currentSourceDir)
-{
-  if (!cmSystemTools::FileIsFullPath(path)) {
-    return cmSystemTools::CollapseFullPath(currentSourceDir + "/" + path);
-  }
-  return cmSystemTools::CollapseFullPath(path);
-}
-
 std::vector<std::string> prepareFilesPathsForTree(
   const std::vector<std::string>& filesPaths,
   const std::string& currentSourceDir)
@@ -80,9 +71,11 @@ std::vector<std::string> prepareFilesPathsForTree(
   prepared.reserve(filesPaths.size());
 
   for (auto const& filePath : filesPaths) {
+    std::string fullPath =
+      cmSystemTools::CollapseFullPath(filePath, currentSourceDir);
     // If provided file path is actually not a file, silently ignore it.
-    if (cmSystemTools::FileExists(filePath, /*isFile=*/true)) {
-      prepared.push_back(prepareFilePathForTree(filePath, currentSourceDir));
+    if (cmSystemTools::FileExists(fullPath, /*isFile=*/true)) {
+      prepared.emplace_back(std::move(fullPath));
     }
   }