Rewrite fetch.sh in Perl.

Rewrite fetch.sh in Perl.

Using HTTP::Tiny to talk to the network, instead of wget, means that
we can make just one TCP connection to git.savannah.gnu.org to do the
whole job, which is quite a bit faster.  It should also be more
robust against weird characters in filenames / URLs and stuff.

The script has a higher requirement for Perl than is the standard in
autoconf -- 5.14 (first version with HTTP::Tiny), with IO::Socket::SSL
installed -- but that’s ok, I hope, because it’s maintainer-only and
not installed.

fetch.sh was the sole user of move-if-change, and the Perl script does
that job itself, but I left move-if-change in build-aux and on the fetch
list anyway, in case we discover another use for it in the future.

* build-aux/fetch.sh: Replace with...
* build-aux/fetch.pl: ... reimplementation in Perl.
* cfg.mk (fetch): Update to match.