mingw: improve performance of mingw_unlink()

Update mingw_unlink() to first try to delete the file with existing
permissions before trying to force it.

Windows throws an error when trying to delete a read-only file.  The
mingw_unlink() compatibility wrapper always tries to _wchmod(666) the
file before calling _wunlink() to avoid that error.  However, since
most files in the worktree are already writable, this is usually
wasted effort.

Update mingw_unlink() to just call DeleteFileW() directly and if that
succeeds return.  If that fails, fall back into the existing code path
to update the permissions and use _wunlink() to get the existing
error code mapping.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>

compat/mingw.c

@@ -508,6 +508,9 @@ int mingw_unlink(const char *pathname)
 	if (xutftowcs_long_path(wpathname, pathname) < 0)
 		return -1;
 
+	if (DeleteFileW(wpathname))
+		return 0;
+
 	do {
 		/* read-only files cannot be removed */
 		_wchmod(wpathname, 0666);