t2003: work around path mangling issue on Windows

MSYS bash considers the part "/g" in the sed expression "s/./=/g" as an
absolute path after an assignment, and mangles it to a C:/something
string. Do not attract bash's attention by avoiding the equals sign.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/checkout-attribute-lookup' into next

* jk/checkout-attribute-lookup:
  t2003: work around path mangling issue on Windows