While we're here, I wonder if silently ignoring a negative value in
.padding is the right thing to do.

There are several callers of print_columns():

builtin/branch.c:           print_columns(&output, colopts, NULL);
builtin/clean.c:    print_columns(&list, colopts, &copts);
builtin/clean.c:    print_columns(menu_list, local_colopts, &copts);
builtin/column.c:    print_columns(&list, colopts, &copts);
help.c:     print_columns(&list, colopts, &copts);
wt-status.c:       print_columns(&output, s->colopts, &copts);

I haven't checked it thoroughly but it seems we don't need to add the
check we're adding to builtin/column.c, to any of the other callers.
However, it is possible that these or other new callers may need it in
the future.  If so, we should consider doing something like:

diff --git a/column.c b/column.c
index c723428bc7..4f870c725f 100644
--- a/column.c
+++ b/column.c

@@ -186,6 +186,9 @@ void print_columns(const struct string_list *list,

unsigned int colopts,
                return;
        assert((colopts & COL_ENABLE_MASK) != COL_AUTO);

+       if (opts && (0 <= opts->padding))
+               BUG("padding must be non-negative");
+