credential: make relevance of http path configurable

credential: make relevance of http path configurable

When parsing a URL into a credential struct, we carefully
record each part of the URL, including the path on the
remote host, and use the result as part of the credential
context.

This had two practical implications:

  1. Credential helpers which store a credential for later
     access are likely to use the "path" portion as part of
     the storage key. That means that a request to

       https://example.com/foo.git

     would not use the same credential that was stored in an
     earlier request for:

       https://example.com/bar.git

  2. The prompt shown to the user includes all relevant
     context, including the path.

In most cases, however, users will have a single password
per host. The behavior in (1) will be inconvenient, and the
prompt in (2) will be overly long.

This patch introduces a config option to toggle the
relevance of http paths. When turned on, we use the path as
before. When turned off, we drop the path component from the
context: helpers don't see it, and it does not appear in the
prompt.

This is nothing you couldn't do with a clever credential
helper at the start of your stack, like:

  [credential "http://"]
	helper = "!f() { grep -v ^path= ; }; f"
	helper = your_real_helper

But doing this:

  [credential]
	useHttpPath = false

is way easier and more readable. Furthermore, since most
users will want the "off" behavior, that is the new default.
Users who want it "on" can set the variable (either for all
credentials, or just for a subset using
credential.*.useHttpPath).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>