t5551: drop curl trace lines without headers

t5551: drop curl trace lines without headers

We pick apart a curl trace, looking for "=> Send header:" and so on, and
matching against an expected set of requests and responses. We remove
"== Info" lines entirely. However, our parser is fooled when running the
test with LIB_HTTPD_SSL on Ubuntu 20.04 (as found in our linux-gcc CI
job), as curl hands us an "Info" buffer with a newline, and we get:

  == Info: successfully set certificate verify locations:
  == Info:   CAfile: /etc/ssl/certs/ca-certificates.crt
    CApath: /etc/ssl/certs
  => Send SSL data[...]

which results in the "CApath" line ending up in the cleaned-up output,
causing the test to fail.

Arguably the tracing code should detect this and put it on two separate
"== Info" lines. But this is actually a curl bug, fixed by their
80d73bcca (tls: provide the CApath verbose log on its own line,
2020-08-18). It's simpler to just work around it here.

Since we are using GIT_TRACE_CURL, every line should just start with one
of "<=", "==", or "=>", and we can throw away anything else. In fact, we
can just replace the pattern for deleting "*" lines. Those were from the
old GIT_CURL_VERBOSE output, but we switched over in 14e24114d9
(t5551-http-fetch-smart.sh: use the GIT_TRACE_CURL environment var,
2016-09-05).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>