Clean up references to Subversion in documentation sources.

Clean up references to Subversion in documentation sources.

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.