sparse-index: pass string length to index_file_exists()

sparse-index: pass string length to index_file_exists()

The call to index_file_exists() in the loop in expand_to_path() passes
the wrong string length.  Let's fix that.

The loop in expand_to_path() searches the name-hash for each
sub-directory prefix in the provided pathname. That is, by searching
for "dir1/" then "dir1/dir2/" then "dir1/dir2/dir3/" and so on until
it finds a cache-entry representing a sparse directory.

The code creates "strbuf path_mutable" to contain the working pathname
and modifies the buffer in-place by temporarily replacing the character
following each successive "/" with NUL for the duration of the call to
index_file_exists().

It does not update the strbuf.len during this substitution.

Pass the patched length of the prefix path instead.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>