indexes: add comment indication location of python27-sys
The indexes module depends directly on the python27-sys crate for low-level
manipulation of Python structures. This is located in a subdir in the cpython
While that works for git repositories, this causes problems when the crate is
vendored, as Cargo is no longer able to find the directory.
Add a comment to the git URL line with the name of the subdirectory. This will
be used by the build script to locate the vendored version of the crate.
Differential Revision: https://phab.mercurial-scm.org/D1712