Change logic of config sections reading

Now ```[buildpackage]```  section (and all other sections) affects ```gbp``` and ```git``` commands. (Before this
section affects only to ```gbp```).
If user want specify different options for ```gbp``` and ```git``` he can add ```[gbp-buildpackage]``` and ```[git-buildpackage]``` sections.

I.e. now
[command] - common options for git-command and gbp command
[gbp-command] - options specific to gbp command
[git-command] - options specific to git-command