var: do not print usage() with a correct invocation

var: do not print usage() with a correct invocation

Before, git-var could print usage() even if the command was invoked
correctly with a variable defined in git_vars -- provided that its
read() function returned NULL.

Now, we only print usage() only if it was called with a logical
variable that wasn't defined -- regardless of read().

Since we now know the variable is valid when we call read_var(), we
can avoid printing usage() here (and exiting with code 129) and
instead exit quietly with code 1. While exiting with a different code
can be a breaking change, it's far better than changing the exit
status more generally from 'failure' to 'success'.

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>