Make Environments not store their own Repo objects

[pv]

Environment only needs to know about a Repo when it is building a
project. Pass the repository in as an argument when needed.

The actual issue this fixes is to avoid cloning the repository when it
is not necessary, for example in 'asv run --python=same'.
(The cloning in this case is a regression introduced in gh-352)

• WIP: may still need some testing

[coveralls]

Coverage decreased (-0.2%) to 70.299% when pulling 8aa4461 on pv:avoid-clone into f5d916c on spacetelescope:master.

[coveralls]

Coverage decreased (-0.1%) to 70.112% when pulling 438dbb8 on pv:avoid-clone into 7c72fc0 on spacetelescope:master.

[coveralls]

Coverage increased (+0.1%) to 70.251% when pulling 01bfa3d on pv:avoid-clone into 1a4afff on spacetelescope:master.

[coveralls]

Coverage decreased (-0.07%) to 70.391% when pulling d8544d6 on pv:avoid-clone into bea2cbd on spacetelescope:master.

[pv]

Merging, should be OK.