credential: do not store credentials received from helpers

The typical call flow for the credential API is something
like:

  1. Network code (like http.c) wants a credential. It calls
     credential_fill() to get it, and one of the two happens:

     a. We call out to any helpers with a "get" request,
	one of which provides the credential.

     b. No helper provides us the credential, so we ask the
        user.

  2. The network code uses the credential. Let's imagine
     that the request completes successfully.

  3. The network code informs the credential subsystem of
     success by calling credential_approve(). The credential
     code then calls out to any helpers with a "store"
     request, so they can optionally store it if they want.

In the case of (1b), this is a good thing. The credential
comes from the user, gets used, and then gets put into
storage for later use again. But in the case of (1a), we end
up feeding the helper with a credential that came from
itself already (or possibly from another helper).

Most of the time, this is harmless and slightly inefficient.
But it has two user-visible impacts:

  1. If you have two helpers, you end up propagating data
     between them. For instance, imagine you have a helper
     "git-credential-wallet" which pulls data from a a
     read-only password wallet. You might configure git like
     this:

       [credential]
		helper = wallet
		helper = cache

     to check the wallet first, and then fall back to asking
     the user and caching the result. But when we do get
     something out of the wallet, git will then ask for it
     to be stored in the wallet (which is a no-op), and the
     cache. So your credential migrates from the wallet into
     the cache, which may not be what you want (e.g.,
     because the wallet implements more strict security
     policies than the cache).

  2. If you use a time-based storage helper like
     "git-credential-cache", every time you run a git
     command which uses the credential, it will also
     re-insert the credential after use, freshening the
     cache timestamp. So the cache will eventually expire N
     time units after the last _use_, not after the time the
     user actually typed the password. This is probably not
     what most users expect or want (and if they do, we
     should do it explicitly by providing an option to
     refresh the timestamp on use).

We can solve this by marking a credential that comes from a
helper, so we don't bother feeding it back to the helpers.
The credential struct already has an "approved" flag so
that we try to store it only once, rather than for each
successful http request. We can use the same flag to
"pre-approve" a credential which comes from a helper, and
enver try to store it at all.
add credential-wrap

This lets you wrap a "real" helper inside a cache.
credential: store and relay unknown credential lines

Just because git itself does not make use of a line does not
mean that another helper might not want to see it.