scripts/get_maintainer.pl: default to not include unspecified tags

This changes the default of the option --git-all-signature-types to be
disabled by default.

The effect being, that only certain (currently Signed-Off-By:, Acked-by:
and Reviewed-By:) tags are used to get adresses of potential maintainers.

Motivated is this change by the desire to not 'spam' people unnecessary: A
Tested-By or a Reported-By is not ment as a hint that those people want
to/are able to review patches to the code in question.

In a quest to find resilient statistics for this i came up with this:

I produced a list of all the tag-signers not already covered with a
signed-off/acked/reviewed tag somewhere in the last year of git history.

Those were 650 addresses of "assumed non-developers".

And to check if those "assumed non-developers" are professional
testers/reporters worth cc'ing, i then counted their total appearences
in the git log:

469 were mentioned only once.
123 were mentioned twice.
38 three times
8 four times
5 six...