Cred helper improvements

[technoweenie]

This PR adds support for per-url credential helpers (#2686) and fixes performance issues discussed in #2690.

• Teach the LFS credential cacher to only call approve the first time. LFS should only ever call git credential approve for a given url/path once in the lifetime of a single LFS command.
• Teach the LFS HTTP client to only call Approve() on the credential helper if it was used for a request. I don't know how other LFS servers work, but GitHub's doesn't require it for transferring specific objects.
• Add approve tracing to LFS so we can confirm a fix.
• Teach LFS to only send path to credential helpers if credential.useHttpPath is enabled.
• If askpass is set, fall back to credential helper if it's misconfigured.
• Check for credential.<url>.usehttppath and credential.<url>.helper

By creating the credential helper per request, any notion of a cache is lost. So, there's still more work to be done in (*Client) getCredentialHelper.

[technoweenie]

Here are some sample builds in case anyone wants to try this out:

git-lfs-darwin-amd64-2.3.4.tar.gz
git-lfs-linux-amd64-2.3.4.tar.gz
git-lfs-windows-amd64-2.3.4.zip

[wouterh-dev]

As mentioned in #2634 (comment) I tested the linux build and confirmed they resolve #2695