emacs: separate history for operations which accept single and multiple tags

Some tag-related operations accept a single tag without prefix
(`notmuch-select-tag-with-completion'), others accept multiple tags
prefixed with '+' or '-' (`notmuch-read-tag-changes').  Before the
change, both functions used a single default minibuffer history.  This
is inconvenient because you have to skip options with incompatible
format when going through the history.  The patch adds separate
history lists for the two functions.  Note that functions that accept
the same input format (e.g. "+", "-", "*") share the history list as
before.