fix(db-functions): acquire a lock around git clone and fetch operations

An initial git clone and especially a git fetch operation can occur
concurrently through multiple packagers and processes. Guard these git
operations with a fetch lock inside the actual git repository as well as
a special lock stored in the shared global lock directory before
starting an initial clone.

Fixes #49

Signed-off-by: Levente Polyak <anthraxx@archlinux.org>