api.py: Adds overlay type checking

Checks have been added to determine whether or not to readd a repo
if the overlay type reported from the remotedb is different from the
overlay type reported by the local db.
overlay.py: adds update() function

To update local database and overlay source urls, this function
has been created to allow the overlay.type functions to update
their source urls in their own methods.
overlay.py: corrects first_src boolean check
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()
reposconf.py WIP: closes set function
repoconfmanager.py WIP: improves error message grammar

Also corrects function indentation.