Merged the following poky fetcher fixes:

Merged the following poky fetcher fixes:

- bitbake/fetch: Allow checking of a single url at a time (massive sstate
  speed improvement)
- bitbake/fetch: Fix handling of mirrors when checking for url validity
- bitbake/fetch: When fetchers return errors, ensure any partial download is
  cleared
- bitbake/fetch: Make URL checking slightly less verbose (distracting with the
  sstate code)
- bitbake/fetch/git: Ensure fullclone repositories are fully fetched
- bitbake/fetcher: Deal with a ton of different bugs
- bitbake/fetch/git.py: Fix git fetcher to correctly use mirror tarballs
- bitbake/fetch/__init__.py: Abstract mirror variable handling and enhance to
  accept \n delimitation in variables
- bitbake/fetch: fix logic to prevent fetches when the file already exists
- bitbake/fetch: ensure the go() method completes when not using premirrors
- Export ALL_PROXY for bitbake commands
- bitbake: Add proxy variables to standard export list
- bitbake/fetch/git: fix try_premirror() definition .Method takes 4
  parameters.
- bitbake/fetch: fix by moving try_premirror() to the right place
- bitbake/fetch: add try_premirror method and special case git fetcher
- fetch: fix setting of localpath in SRC_URI parameters
- bitbake/fetch: ensure the mirrored repository is updated as required
- bitbake/git: define a forcefetch method
- bitbake/fetch: Respect forcefetch even when pulling from a mirror
- bitbake/git.py: Make sure different branches can have different revisions
  without triggering build count increases
- bitbake/git.py: Allow tracking of branches in SRC_URI without cloning for
  use with fullclone
- bitbake/git.py: Make sure a full clone checkout always updates

Signed-off-by: Chris Larson <chris_larson@mentor.com>