gitcredentials(7): clarify quoting of helper examples

gitcredentials(7): clarify quoting of helper examples

We give several helper config examples, but don't make clear that these
are raw values. It's up to the user to add the appropriate quoting to
put them into a config file (either by running with "git config" and
quoting against the shell, or by adding double-quotes as appropriate
within the git-config file).

Let's flesh them out as full config blocks, which makes the syntax more
clear (and makes it possible for people to just cut-and-paste them as a
starting point). I added double-quotes to any values larger than a
single word. That isn't strictly necessary in all cases, but it
sidesteps explaining the rules about exactly when you need to quote a
value.

The existing quotes can be converted to single-quotes in one instance,
and backslash-esccaped in the other. I also swapped out backticks for
our preferred $().

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