[tree] rename bit to avoid confusion with const TFile

as suggested by pcanal

Author: ferdymercury

io/io/inc/TFile.h

@@ -277,7 +277,7 @@ class TFile : public TDirectoryFile {
       // Set this bit to block an unwanted automatic call to TTree::ChangeFile
       // e.g. during TFileMerger operations (bypassing any set MaxTreeSize)
       // This is a transient bit and has no relevance for I/O.
-      kCannotChange  = BIT(18)
+      kCancelTTreeChangeRequest = BIT(18)
    };
    enum ERelativeTo { kBeg = 0, kCur = 1, kEnd = 2 };
    enum { kStartBigFile  = 2000000000 };

io/io/src/TFileMerger.cxx

@@ -978,7 +978,7 @@ Bool_t TFileMerger::PartialMerge(Int_t in_type)
    }
 
    fOutputFile->SetBit(kMustCleanup);
-   fOutputFile->SetBit(TFile::kCannotChange);
+   fOutputFile->SetBit(TFile::kCancelTTreeChangeRequest);
 
    TDirectory::TContext ctxt;
 
@@ -1040,7 +1040,7 @@ Bool_t TFileMerger::PartialMerge(Int_t in_type)
       Clear();
    } else {
       fOutputFile->ResetBit(kMustCleanup);
-      fOutputFile->ResetBit(TFile::kCannotChange);
+      fOutputFile->ResetBit(TFile::kCancelTTreeChangeRequest);
       SafeDelete(fOutputFile);
    }
    return result;

tree/tree/src/TTree.cxx

@@ -2744,7 +2744,7 @@ TFile* TTree::ChangeFile(TFile* file)
 {
    // Changing file clashes with the design of TMemFile and derivates, see #6523,
    // as well as with TFileMerger operations, see #6640.
-   if ((dynamic_cast<TMemFile *>(file)) || file->TestBit(TFile::kCannotChange))
+   if ((dynamic_cast<TMemFile *>(file)) || file->TestBit(TFile::kCancelTTreeChangeRequest))
       return file;
    file->cd();
    Write();