fetch-pack: redact packfile urls in traces

fetch-pack: redact packfile urls in traces

In some setups, packfile uris act as bearer token. It is not
recommended to expose them plainly in logs, although in special
circunstances (e.g. debug) it makes sense to write them.

Redact the packfile URL paths by default, unless the GIT_TRACE_REDACT
variable is set to false. This mimics the redacting of the Authorization
header in HTTP.

Signed-off-by: Ivan Frade <ifrade@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>