Add support for building from git.

Author:    Brian Bockelman <bbockelm@cse.unl.edu>

git-svn-id: file:///p/vdt/workspace/svn/software/osg-build/trunk@17356 4e558342-562e-0410-864c-e07659590f8c
Add --svn option back for compatibility (as a synonym for --vcs).

git-svn-id: file:///p/vdt/workspace/svn/software/osg-build/trunk@17357 4e558342-562e-0410-864c-e07659590f8c
Correctly set the tag (for koji add-pkg) on alternate repos.

Author:    Brian Bockelman <bbockelm@cse.unl.edu>

git-svn-id: file:///p/vdt/workspace/svn/software/osg-build/trunk@17358 4e558342-562e-0410-864c-e07659590f8c
Fix check for git when git is not installed on the host.

Author:    Brian Bockelman <bbockelm@cse.unl.edu>

git-svn-id: file:///p/vdt/workspace/svn/software/osg-build/trunk@17359 4e558342-562e-0410-864c-e07659590f8c