cmd/go: "Go get" should look locally for git config rules

[ajzaff]

Using Golang 1.7.5

Background: go get will fail for private repos which require a username and password to pull. One workaround is adding some configuration in ~/.gitconfig:

[url "ssh://[email protected]/"]
    insteadOf = https://github.PRIVATE.com/

or:

[url "https://${TOKEN}:[email protected]/"]
    insteadOf = https://github.PRIVATE.com/

This is nice. However, go get will ignore the same local setting in ./.git/config in the currect directory.

Story: As a CLI user under Git, I should be able to define --local rules for geting private packages from Github

To duplicate this issue:

0. configure an ssh private key with read access to the private repo
1. go get github.PRIVATE.com/MY/REPO, expect fail
2. git config --local url."ssh://[email protected]/".insteadOf "https://github.PRIVATE.com/"
3. go get github.PRIVATE.com/MY/REPO, expect fail
4. git config --global url."ssh://[email protected]/".insteadOf "https://github.PRIVATE.com/"
5. go get github.PRIVATE.com/MY/REPO, expect to work