notmuch-hello: Fix a sign error when computing number of padding spaces.

notmuch-hello: Fix a sign error when computing number of padding spaces.

After the previous commit, toggling the visibility of tags could
result in notmuch-hello aborting with:

	Wrong type argument: wholenump, -1

At least, the error only occurred for me when making tags visible. But
that may be because my longest tag name is longer than my longest
saved-search name.