On Cygwin, use symbolic ref, not a symbolic link, to express .git/HEAD

H. Peter Anvin says that Samba "promotes" symlinks to hardlinks while
Cygwin itself uses .lnk files to emulate symlinks.  Avoid using symbolic
link for .git/HEAD on Cygwin.

This does not help the symlinks recorded in trees as user data, but
at least we do not use them for our own bookkeeping.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -183,6 +183,7 @@ ifeq ($(shell uname -o),Cygwin)
 	NEEDS_LIBICONV = YesPlease
 	NO_IPV6 = YesPlease
 	X = .exe
+	PLATFORM_DEFINES += -DUSE_SYMLINK_HEAD=0
 endif
 ifneq (,$(findstring arm,$(shell uname -m)))
 	ARM_SHA1 = YesPlease