win32: use our own dirent.h

The mingw-runtime implemenation of opendir, readdir and closedir
sets errno to 0 on success, something that POSIX explicitly
forbids. 3ba7a06 ("A loose object is not corrupt if it cannot be
read due to EMFILE") introduce a dependency on this behaviour,
leading to a broken "git clone" on Windows.

compat/mingw.c contains an implementation of readdir, and
compat/msvc.c contains implementations of opendir and closedir.

Move these to compat/win32/dirent.[ch], and change to our own DIR
structure at the same time.

This provides a generic Win32-implementation of opendir, readdir
and closedir which works on both MinGW and MSVC and does not reset
errno, and as a result git clone is working again on Windows.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -319,35 +319,6 @@ int main(int argc, const char **argv) \
 } \
 static int mingw_main(c,v)
 
-#ifndef NO_MINGW_REPLACE_READDIR
-/*
- * A replacement of readdir, to ensure that it reads the file type at
- * the same time. This avoid extra unneeded lstats in git on MinGW
- */
-#undef DT_UNKNOWN
-#undef DT_DIR
-#undef DT_REG
-#undef DT_LNK
-#define DT_UNKNOWN	0
-#define DT_DIR		1
-#define DT_REG		2
-#define DT_LNK		3
-
-struct mingw_dirent
-{
-	long		d_ino;			/* Always zero. */
-	union {
-		unsigned short	d_reclen;	/* Always zero. */
-		unsigned char   d_type;		/* Reimplementation adds this */
-	};
-	unsigned short	d_namlen;		/* Length of name in d_name. */
-	char		d_name[FILENAME_MAX];	/* File name. */
-};
-#define dirent mingw_dirent
-#define readdir(x) mingw_readdir(x)
-struct dirent *mingw_readdir(DIR *dir);
-#endif // !NO_MINGW_REPLACE_READDIR
-
 /*
  * Used by Pthread API implementation for Windows
  */