git: sync_fetch should fetch HEAD ref

hooks: expose few extra variables from hook calls

dependencies: bumped few libraries. - atomicwrites==1.2.1 - attrs==18.2.0 - dogpile.cache==0.6.7 - psutil==5.4.7 - pathlib2==2.3.2 - subprocess32==3.5.2 - gevent==1.3.6 - greenlet==0.4.15 - pytest==3.8.1 - py==1.6.0 - pytest-cov==2.6.0 - pytest-timeout==1.3.2 - coverage==4.5.1

hooks: store hook type for extensions.

git: use nicer failed command debug. Allows verifiy the directory, and env options that might change call behaviour.

hooks: handle errors before trying to fetch the output.

git: use --force for fetch command so we can force update non-fast-forward refs from the source

git: allow checkout in pull command

git: rename fetch into pull because this is what it actually does.

git: use new sync_fetch that uses git subcommand instead of fragile dulwich code.