Update several references to "subversion" to be git or github.

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23783 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Retire Nightly/weekend as it is no longer used.

Discussed w/ @vasslitvinov

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23784 3a8e244f-b0f2-452b-bcba-4c88e055c3ca