add --edit: truncate the patch file

If there is already a .git/ADD_EDIT.patch file, we fail to truncate it
properly, which could result in very funny errors.

Let's just truncate it and not worry about it too much.

Reported by J Wyman.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/add.c

@@ -239,7 +239,7 @@ static int edit_patch(int argc, const char **argv, const char *prefix)
 	rev.diffopt.output_format = DIFF_FORMAT_PATCH;
 	rev.diffopt.use_color = 0;
 	rev.diffopt.flags.ignore_dirty_submodules = 1;
-	out = open(file, O_CREAT | O_WRONLY, 0666);
+	out = open(file, O_CREAT | O_WRONLY | O_TRUNC, 0666);
 	if (out < 0)
 		die(_("Could not open '%s' for writing."), file);
 	rev.diffopt.file = xfdopen(out, "w");