t1300-repo-config: two new config parsing tests.

- correctly insert a new variable into a section that only
  contains a single (different) variable.

- correctly insert a new section that matches the initial
  substring of an existing section.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>