git_config: add SetBoolean helper

A little sugar simplifies the code a bit.

Change-Id: Ie2b8a965faa9f9ca05c7be479d03e8e073cd816d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296522


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>