Reintroduce ROOT_SVN_REVISION for backward compatibility.

ROOT_SVN_REVISION is re-introduced for backward compatibility. However
it is fixed at the last recorded svn revision for this branch 49361.
It also fixes a typo in core/base/inc/RVersion.h that fixes ROOT-5129.
Note that the PROOF code using TROOT::GetSvnRevision() still has to be
changed to use TROOT::GetGitCommit().
Add --git-dir option to git for out of source build. Fixes ROOT-5127.