fsck: reject URL with empty host in .gitmodules

fsck: reject URL with empty host in .gitmodules

Git's URL parser interprets

	https:///example.com/repo.git

to have no host and a path of "example.com/repo.git".  Curl, on the
other hand, internally redirects it to https://example.com/repo.git.  As
a result, until "credential: parse URL without host as empty host, not
unset", tricking a user into fetching from such a URL would cause Git to
send credentials for another host to example.com.

Teach fsck to block and detect .gitmodules files using such a URL to
prevent sharing them with Git versions that are not yet protected.

A relative URL in a .gitmodules file could also be used to trigger this.
The relative URL resolver used for .gitmodules does not normalize
sequences of slashes and can follow ".." components out of the path part
and to the host part of a URL, meaning that such a relative URL can be
used to traverse from a https://foo.example.com/innocent superproject to
a https:///attacker.example.com/exploit submodule. Fortunately,
redundant extra slashes in .gitmodules are rare, so we can catch this by
detecting one after a leading sequence of "./" and "../" components.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>