This commit changes git/cloner.go from cloning the whole history
and then checking out the desired ref to a implementation that
only downloads the history for the desired ref.
It does so by first initializing an empty repository, setting the
source repository as a remote, fetching just the desired ref and
then hard resetting the empty local repo to that ref.
This reduces the time it takes to build the multibases example
as a remote base at ref v2.0.3 from an avg of 8s with the
current implementation to an avg of 2s out of 10 runs each, by
drastically decreasing the data transferred.
The improvement should increase as repositories grow.
This PR closes a TODO in the fileLoader type to
replace the string representing a git repo url
with a struct called holding validated fields.
New struct is called RepoSpec.
It holds what we need in coming PRs for remote
repo base containment checks, and will also allow
the overlay cycle prevention code to take into
account different host schemes or branches.
The struct is in a new file called repospec.go.
The new file is mostly just a code move of url
parsing code that was in cloner.go. Git "blame"
history preserved through the move.
No change in execution (see final commit by itself).
