editing: avoid creating blank lines when using autoindent

When Enter is pressed while the cursor is exactly on the current
indent width, remove the blank characters on that line to avoid
creating a line that consists only of trailing whitespace.

(When Enter is pressed somewhere in the middle of the blanks,
however, the whitespace is preserved.)
Suggested-by: Florian Zeitz <florob@babelmonkeys.de>

src/text.c

@@ -768,11 +768,13 @@ void do_undo(void)
 	    break;
 	}
 	undidmsg = _("line break");
+	size_t from_x = (u->begin == 0) ? 0 : u->mark_begin_x;
+	size_t to_x = (u->begin == 0) ? u->mark_begin_x : u->begin;
 	f->data = charealloc(f->data, strlen(f->data) +
-				strlen(&f->next->data[u->mark_begin_x]) + 1);
-	strcat(f->data, &f->next->data[u->mark_begin_x]);
+				strlen(&u->strdata[from_x]) + 1);
+	strcat(f->data, &u->strdata[from_x]);
 	unlink_node(f->next);
-	goto_line_posx(u->lineno, u->begin);
+	goto_line_posx(u->lineno, to_x);
 	break;
 #ifdef ENABLE_COMMENT
     case COMMENT:
@@ -982,12 +984,11 @@ void do_enter(void)
 {
     filestruct *newnode = make_new_node(openfile->current);
     size_t extra = 0;
+    bool allblanks = FALSE;
 
     assert(openfile->current != NULL && openfile->current->data != NULL);
 
 #ifndef NANO_TINY
-    add_undo(ENTER);
-
     if (ISSET(AUTOINDENT)) {
 	extra = indent_length(openfile->current->data);
 
@@ -995,6 +996,8 @@ void do_enter(void)
 	 * indentation to the current x position. */
 	if (extra > openfile->current_x)
 	    extra = openfile->current_x;
+	else if (extra == openfile->current_x)
+	    allblanks = TRUE;
     }
 #endif
     newnode->data = charalloc(strlen(openfile->current->data +
@@ -1005,13 +1008,19 @@ void do_enter(void)
     if (ISSET(AUTOINDENT)) {
 	/* Copy the whitespace from the current line to the new one. */
 	strncpy(newnode->data, openfile->current->data, extra);
-	openfile->totsize += extra;
+	/* If there were only blanks before the cursor, trim them. */
+	if (allblanks)
+	    openfile->current_x = 0;
+	else
+	    openfile->totsize += extra;
     }
 #endif
 
     null_at(&openfile->current->data, openfile->current_x);
 
 #ifndef NANO_TINY
+    add_undo(ENTER);
+
     /* Adjust the mark if it was on the current line after the cursor. */
     if (openfile->mark_set && openfile->current == openfile->mark_begin &&
 		openfile->current_x < openfile->mark_begin_x) {