[tree] Fix invalid macro names when absolute filepath is passed

Fixes root-project#16562

tree/treeviewer/src/TTreeViewer.cxx

@@ -226,6 +226,8 @@ or :
 #include "strlcpy.h"
 #include "snprintf.h"
 
+#include <filesystem>
+
 #ifdef WIN32
 #include "TWin32SplashThread.h"
 #endif
@@ -2741,10 +2743,8 @@ void TTreeViewer::SaveSource(const char* filename, Option_t *)
  }
  // Write macro header and date/time stamp
  TDatime t;
- TString sname(fname);
- sname = sname.ReplaceAll(".C", "");
  out <<"void open_session(void *p = 0);"<<std::endl<<std::endl;
- out <<"void "<<sname.Data()<<"() {"<<std::endl;
+ out <<"void "<<std::filesystem::path(fname.Data()).stem()<<"() {"<<std::endl;
  out <<"//=========Macro generated by ROOT version"<<gROOT->GetVersion()<<std::endl;
  out <<"//=========for tree "<<quote<<fTree->GetName()<<quote<<" ("<<t.AsString()<<")"<<std::endl;
  out <<"//===This macro can be opened from a TreeViewer session after loading"<<std::endl;