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.

Of course, this file should not be left lying around. But at least in
one case, there was a stale copy, larger than the current diff. So the
result was a corrupt diff.

Let's just truncate the file when we write it and not worry about it too
much.

Reported by J Wyman.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[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");