t5510: replace 'origin' with URL more carefully

t5510: replace 'origin' with URL more carefully

The many test_configured_prune tests in t5510-fetch.sh test many
combinations of --prune, --prune-tags, and using 'origin' or an explicit
URL. Some machinery was introduced in e1790f9245f (fetch tests: fetch
<url> <spec> as well as fetch [<remote>], 2018-02-09) to replace
'origin' with this explicit URL. This URL is a "file:///" URL for the
root of the $TRASH_DIRECTORY.

However, if the current build tree has an '@' symbol, the
replacement using perl fails. It drops the '@' as well as anything
else in that directory name.  You can observe this locally by
cloning git.git into a "victim@03" directory and running the test
script.

As we are writing in Perl anyway, pass in the shell variables involved
to the script as arguments and perform necessary string transformations
inside it, instead of assuming that it is sufficient to enclose the
$remote_url variable inside a pair of single quotes.

Reported-by: Randall Becker <rsbecker@nexbridge.com>
Original-patch-by: Derrick Stolee <derrickstolee@github.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>