chainlint: reduce annotation noise-factor

When chainlint detects a problem in a test definition, it highlights the
offending code with a "?!...?!" annotation. The rather curious "?!"
decoration was chosen to draw the reader's attention to the problem area
and to act as a good "needle" when using the terminal's search feature
to "jump" to the next problem.

Later, chainlint learned to color its output when sent to a terminal.
Problem annotations are colored with a red background which stands out
well from surrounding text, thus easily draws the reader's attention.
Together with the preceding change which gave all problem annotations a
uniform "LINT:" prefix, the noisy "?!" decoration has become superfluous
as a search "needle" so omit it when output is colored.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>