tweaks: simplify the undoing and redoing of an <Enter>

I don't know what I was thinking three years ago with this convoluted
'from_x' and 'to_x'.  When undoing an <Enter>, the cursor always needs
to be placed back at the 'head' point.  And the data always needs to be
copied starting from 'tail_x' (skipping any whitespace that autoindent
might have added).

When redoing an <Enter>, there is no need to reallocate and copy the
data of the line, it is enough to just clip it at the original cursor
position.  (This wastes some memory, but... how often does one redo
an <Enter>?  So, conciseness of the code is preferable.)

src/text.c

@@ -516,7 +516,6 @@ void do_undo(void)
 	linestruct *f = NULL, *t = NULL;
 	linestruct *oldcutbuffer;
 	char *data, *undidmsg = NULL;
-	size_t from_x, to_x;
 
 	if (u == NULL) {
 		statusbar(_("Nothing to undo"));
@@ -544,14 +543,12 @@ void do_undo(void)
 		break;
 	case ENTER:
 		undidmsg = _("line break");
-		from_x = (u->head_x == 0) ? 0 : u->tail_x;
-		to_x = (u->head_x == 0) ? u->tail_x : u->head_x;
 		f->data = charealloc(f->data, strlen(f->data) +
-								strlen(&u->strdata[from_x]) + 1);
-		strcat(f->data, &u->strdata[from_x]);
+								strlen(&u->strdata[u->tail_x]) + 1);
+		strcat(f->data, &u->strdata[u->tail_x]);
 		unlink_node(f->next);
 		renumber_from(f);
-		goto_line_posx(u->head_lineno, to_x);
+		goto_line_posx(u->head_lineno, u->head_x);
 		break;
 	case BACK:
 	case DEL:
@@ -718,11 +715,9 @@ void do_redo(void)
 		break;
 	case ENTER:
 		redidmsg = _("line break");
+		f->data[u->head_x] = '\0';
 		shoveline = make_new_node(f);
 		shoveline->data = copy_of(u->strdata);
-		data = measured_copy(f->data, u->head_x);
-		free(f->data);
-		f->data = data;
 		splice_node(f, shoveline);
 		renumber_from(shoveline);
 		goto_line_posx(u->head_lineno + 1, u->tail_x);