Add compat/fopen.c which returns NULL on attempt to open directory

Some systems do not fail as expected when fread et al. are called on
a directory stream. Replace fopen on such systems which will fail
when the supplied path is a directory.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -204,6 +204,11 @@ void *gitmemmem(const void *haystack, size_t haystacklen,
                 const void *needle, size_t needlelen);
 #endif
 
+#ifdef FREAD_READS_DIRECTORIES
+#define fopen(a,b) git_fopen(a,b)
+extern FILE *git_fopen(const char*, const char*);
+#endif
+
 #ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 1)
 #define HAVE_STRCHRNUL