config: disallow relative include paths from blobs

config: disallow relative include paths from blobs

When we see a relative config include like:

  [include]
  path = foo

we make it relative to the containing directory of the file
that contains the snippet. This makes no sense for config
read from a blob, as it is not on the filesystem.  Something
like "HEAD:some/path" could have a relative path within the
tree, but:

  1. It would not be part of include.path, which explicitly
     refers to the filesystem.

  2. It would need different parsing rules anyway to
     determine that it is a tree path.

The current code just uses the "name" field, which is wrong.
Let's split that into "name" and "path" fields, use the
latter for relative includes, and fill in only the former
for blobs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>