Use sha1_file.c's mkdir-like routine in apply.c.

Use sha1_file.c's mkdir-like routine in apply.c.

As far as I can see, create_subdirectories() in apply.c just
duplicates the functionality of safe_create_leading_directories() from
sha1_file.c.  The former has a warm, fuzzy const parameter, but that's
not important.

The potential problem with EEXIST and creating directories should
never occur here, but will be removed by future
safe_create_leading_directories() changes.  Other uses of EEXIST in
apply.c should be fine barring intentionally malicious behavior.

Signed-off-by: Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>