transport.c: avoid "whitelist"

transport.c: avoid "whitelist"

The word "whitelist" has cultural implications that are not inclusive.
Thankfully, it is not difficult to reword and avoid its use.

The GIT_ALLOW_PROTOCOL environment variable was referred to as a
"whitelist", but the word "allow" is already part of the variable.
Replace "whitelist" with "allow_list" in these cases to demonstrate that
we are processing a list of allowed protocols.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>