From Sergei Linev:

1) New fMotherDir memeber was added to indicate to which directory and file (via dir->GetFile()) key belongs to.
   Mother directory must be specified in constructor, therefore all consructors get additional argument - mother
   directory
2) New method Build() was added to initialize members of TKey class. It is called from every constructor of
   TKey class.
3) Everywhere gFile and gDirectory were replaced by GetMotherDir() & GetFile(). Two methods: TKey::WriteFile()
   and TKey::Create() gets new argument - TFile* f, indicating which file should be used. By default,
   file from mother directory is used, but for some cases in tree other file pointer specified directly.
4) In TKey::ReadBuffer() sometimes key will be added to gDirectory. For normal cases it should not happen,
   therefore I extract main code to TKey::ReadKeyBuffer() and call it from the places where I know no
   any gDirectory->AppendKey should be called. For instance, when you store streamerinfos. Functionality of
   TKey::ReadBuffer() was not changed. Actually, I did not found a situation, when this function called
   and code with gDirectory->AppendKey() is activated (I check with stress and roottest).
`


git-svn-id: http://root.cern.ch/svn/root/trunk@13912 27541ba8-7e3a-0410-8455-c3a389f83636