clone: tighten "local paths with colons" check a bit

clone: tighten "local paths with colons" check a bit

commit 6000334 (clone: allow cloning local paths with colons in them -
2013-05-04) made it possible to specify a path that has colons in it
without file://, e.g. ../foo:bar/somewhere. But the check was a bit
sloppy.

Consider the url '[foo]:bar'. The '[]' unwrapping code will turn the
string to 'foo\0:bar'. In effect this new string is the same as
'foo/:bar' in the check "path < strchrnul(host, '/')", which mistakes
it for a local path (with '/' before the first ':') when it's actually
not.

So disable the check for '/' before ':' when the URL has been mangled
by '[]' unwrapping.

[jn: with tests from Jeff King]

Noticed-by: Morten Stenshorne <mstensho@opera.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>