handle detached git on restore

[freeformz]

If the git repo is detached, we need to determine the default remote
branch before attempting to donwload (can't pull into a repo in detached
state).

This attempts to use "git remote show origin" to have the origin tell us
what the default branch is.

PS: this may not work with old (~3 year old git servers).

[freeformz]

This is because of this: golang/go@4220659

This doesn't "fix" anything for non godep users, just allows godep restore to work properly.

[freeformz]

This came up in #453