Change the prefix for exact match queries

PR beetbox#4251 added exact match queries, which are great, but it was
subsequently pointed out that the `~` query prefix was already in use:
beetbox#4251 (comment)

So this changes the prefix from `~` to `=~`. A little longer, but
hopefully it makes the relationship to the similarly-new `=` prefix obvious.

# Conflicts:
#	docs/changelog.rst

beets/library.py

@@ -1392,7 +1392,7 @@ def parse_query_parts(parts, model_cls):
     # Get query types and their prefix characters.
     prefixes = {
         ':': dbcore.query.RegexpQuery,
-        '~': dbcore.query.StringQuery,
+        '=~': dbcore.query.StringQuery,
         '=': dbcore.query.MatchQuery,
     }
     prefixes.update(plugins.queries())

docs/reference/query.rst

@@ -93,15 +93,17 @@ backslashes are not part of beets' syntax; I'm just using the escaping
 functionality of my shell (bash or zsh, for instance) to pass ``the rebel`` as a
 single argument instead of two.
 
+.. _exact-match:
+
 Exact Matches
 -------------
 
 While ordinary queries perform *substring* matches, beets can also match whole
-strings by adding either ``=`` (case-sensitive) or ``~`` (ignore case) after the
-field name's colon and before the expression::
+strings by adding either ``=`` (case-sensitive) or ``=~`` (ignore case) after
+the field name's colon and before the expression::
 
     $ beet list artist:air
-    $ beet list artist:~air
+    $ beet list artist:=~air
     $ beet list artist:=AIR
 
 The first query is a simple substring one that returns tracks by Air, AIR, and
@@ -112,16 +114,16 @@ returns tracks by AIR only.
 
 Exact matches may be performed on phrases as well::
 
-    $ beet list artist:~"dave matthews"
+    $ beet list artist:=~"dave matthews"
     $ beet list artist:="Dave Matthews"
 
 Both of these queries return tracks by Dave Matthews, but not by Dave Matthews
 Band.
 
 To search for exact matches across *all* fields, just prefix the expression with
-a single ``=`` or ``~``::
+a single ``=`` or ``=~``::
 
-    $ beet list ~crash
+    $ beet list =~crash
     $ beet list ="American Football"
 
 .. _regex: