text: allow unindenting when not all lines are indented

And also allow it when lines are only partially indented.

This makes it possible to equalize the indentations of (accidentally)
unevenly indented lines: by first fully unindenting a group of lines,
and then reindenting them to the desired amount.
Suggested-by: Liu Hao <lh_mouse@126.com>

src/text.c

@@ -347,23 +347,22 @@ void do_indent(void)
     refresh_needed = TRUE;
 }
 
-/* If the given text starts with a tab's worth of whitespace, return the
- * number of bytes this whitespace occupies.  Otherwise, return zero. */
+/* Return the number of bytes of whitespace at the start of the given text,
+ * but at most a tab's worth. */
 size_t length_of_white(const char *text)
 {
-    size_t bytes_of_white = 1;
+    size_t bytes_of_white = 0;
 
     while (TRUE) {
 	if (*text == '\t')
-	    return bytes_of_white;
+	    return ++bytes_of_white;
 
 	if (*text != ' ')
-	    return 0;
+	    return bytes_of_white;
 
-       if (bytes_of_white == tabsize)
+       if (++bytes_of_white == tabsize)
 	    return tabsize;
 
-	bytes_of_white++;
 	text++;
     }
 }
@@ -414,15 +413,16 @@ void do_unindent(void)
 	bot = top;
     }
 
-    /* If any of the lines cannot be unindented and does not consist of
-     * only whitespace, we don't change anything. */
+    /* Check if there is a line that can be unindented. */
     for (line = top; line != bot->next; line = line->next) {
-	if (length_of_white(line->data) == 0 && !white_string(line->data)) {
-	    statusline(HUSH, _("Can unindent only by a full tab size"));
-	    return;
-	}
+	if (length_of_white(line->data) != 0)
+	   break;
     }
 
+    /* If none of the lines can be unindented, there is nothing to do. */
+    if (line == bot->next)
+	return;
+
     add_undo(UNINDENT);
 
     /* Go through each of the lines and remove their leading indent. */