link_alt_odb_entries(): use string_list_split_in_place()

link_alt_odb_entries(): use string_list_split_in_place()

Change link_alt_odb_entry() to take a NUL-terminated string instead of
(char *, len).  Use string_list_split_in_place() rather than inline
code in link_alt_odb_entries().

This approach saves some code and also avoids the (probably harmless)
error of passing a non-NUL-terminated string to is_absolute_path().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>