git.py: moves fix_git_source() function

This function will also be used by the git update() function
that will be added.
git.py: adds update() function

This function will call git remote set-url, replacing the old url
with the new provided by overlay.update().
overlay.py: removes indexing of sources object in update()
db.py: adds update() function
api.py: incorporates db.update() in api.sync()
api.py: closes interpolation warning message
reposconf.py WIP: closes set function
config.py: Changes require_repoconfig from bool to string
repoconfmanager.py WIP: improves error message grammar

Also corrects function indentation.