Don't add a target to its own also_make list

* src/dep.h (copy_dep): Make a copy of one struct dep.
* src/misc.c (copy_dep): Implement the new function.
(copy_dep_chain): Call copy_dep() for each dep in the chain.
* src/read.c (record_files): Write our own copy loop and omit the
current file from its own also_make list.  Since we don't keep
the function's also_make, free it.