Fix exponential regex performance issue

filter_leading_non_json_lines effectively does

re.match(".*\w+=\w+.*", line)

for every line of output. This has abysmal performance in case of large
Base64-encoded data (which ultimately does not match the regex but does
match the .*\w+= part) as returned e.g. by the template module (diffs).

Replacing the match with

re.search("\w=\w", line)

drops the complexity back to linear, and actually usable with large
diffs from the template module (a 150 KB Base64 diff kept Ansible
spinning at 100% cpu for minutes).

Also, check the easy cases (line.startswith) first while we're here.

Closes: #8932