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.

emacs/notmuch.el

@@ -76,6 +76,14 @@ For example:
 (defvar notmuch-query-history nil
   "Variable to store minibuffer history for notmuch queries")
 
+(defvar notmuch-select-tag-history nil
+  "Variable to store minibuffer history for
+`notmuch-select-tag-with-completion' function.")
+
+(defvar notmuch-read-tag-changes-history nil
+  "Variable to store minibuffer history for
+`notmuch-read-tag-changes' function.")
+
 (defun notmuch-tag-completions (&optional search-terms)
   (split-string
    (with-output-to-string
@@ -86,7 +94,7 @@ For example:
 
 (defun notmuch-select-tag-with-completion (prompt &rest search-terms)
   (let ((tag-list (notmuch-tag-completions search-terms)))
-    (completing-read prompt tag-list)))
+    (completing-read prompt tag-list nil nil nil 'notmuch-select-tag-history)))
 
 (defun notmuch-read-tag-changes (&optional initial-input &rest search-terms)
   (let* ((all-tag-list (notmuch-tag-completions))
@@ -106,7 +114,8 @@ For example:
 	    (define-key map " " 'self-insert-command)
 	    map)))
     (delete "" (completing-read-multiple "Tags (+add -drop): "
-		tag-list nil nil initial-input))))
+		tag-list nil nil initial-input
+		'notmuch-read-tag-changes-history))))
 
 (defun notmuch-update-tags (tags tag-changes)
   "Return a copy of TAGS with additions and removals from TAG-CHANGES.