prefs: fix saving repo-specific configuration

Set the source to 'repo' for the "Current Repository" tab to
ensure that we save configuration values to the local git
repository's .git/config.  The resulting logic for displaying
the values ended up being simpler since it was originally
papering over the logic typo in the constructor.

Closes #174

Reported-by: @scit2010 via github.com
Signed-off-by: David Aguilar <davvid@gmail.com>