Clean up references to SVN in in the GCC docs, redirecting to Git
documentation as appropriate.

Where references to "the source code repository" rather than a
specific VCS make sense, I have used them. You might, after
all, change VCSes again someday.

I have not modified either generated HTML files nor maintainer scripts.
These changes should be complete with repect to the documentation tree.

2020-01-19  Eric S. Raymond <esr@thyrsus.com>

	gcc/
	* doc/contribute.texi: Update for SVN -> Git transition.
	* doc/install.texi: Likewise.

	libstdc++-v3
	* doc/xml/faq.xml: Update for SVN -> Git transition.
	* doc/xml/manual/appendix_contributing.xml: Likewise.
	* doc/xml/manual/status_cxx1998.xml: Likewise.
	* doc/xml/manual/status_cxx2011.xml: Likewise.
	* doc/xml/manual/status_cxx2014.xml: Likewise.
	* doc/xml/manual/status_cxx2017.xml: Likewise.
	* doc/xml/manual/status_cxx2020.xml: Likewise.
	* doc/xml/manual/status_cxxtr1.xml: Likewise.
	* doc/xml/manual/status_cxxtr24733.xml: Likewise.