[cmake] Fix add_version_info_from_vcs git svn version bug.

Summary:
add_version_info_from_vcs was setting SVN_REVISION to the last fetched
svn revision when using git svn instead of the svn revision
corresponding to HEAD.  This leads to conflicts with the definition of
SVN_REVISION in SVNVersion.inc generated by GetSVN.cmake when HEAD is
not the most recently fetched svn revision.

Use 'git svn info' to determine SVN_REVISION when git svn is being used
instead (as is done in GetSVN.cmake).

Reviewers: beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16299

llvm-svn: 258148