tests: assert *.txt SYNOPSIS and -h output

tests: assert *.txt SYNOPSIS and -h output

Add a test to assert basic compliance with the CodingGuidelines in the
SYNOPSIS and builtin -h output. For now we only assert that the "-h"
output doesn't have "\t" characters, as a very basic syntax check.

Subsequent commits will expand on the checks here as various issues
are fixed, but let's first add the test scaffolding.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>