stat() for existence in safe_create_leading_directories()

stat() for existence in safe_create_leading_directories()

Use stat() to explicitly check for existence rather than
relying on the non-portable EEXIST error in sha1_file.c's
safe_create_leading_directories().  There certainly are
optimizations possible, but then the code becomes almost
the same as that in coreutil's lib/mkdir-p.c.

Other uses of EEXIST seem ok.  Tested on Solaris 8, AIX 5.2L,
and a few Linux versions.  AIX has some unrelated (I think)
failures right now; I haven't tried many recent gits there.
Anyone have an old Ultrix box to break everything?  ;)

Also remove extraneous #includes.  Everything's already in
git-compat-util.h, included through cache.h.

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