xapian parser bug?

[David Bremner <david@tethera.net>]

Today we noticed that keywords can't be searched as prefixed terms. Or
that's what it looks like anyway. I tested and, or, and not.

╰─% NOTMUCH_DEBUG_QUERY=y notmuch search 'subject:"and"'
Query string is:
subject:"and"
notmuch search: A Xapian exception occurred
A Xapian exception occurred parsing query: Syntax: <expression> AND <expression>
Query string was: subject:"and"

╰─% NOTMUCH_DEBUG_QUERY=y notmuch search 'subject:"or"' 
Query string is:
subject:"or"
notmuch search: A Xapian exception occurred
A Xapian exception occurred parsing query: Syntax: <expression> OR <expression>
Query string was: subject:"or"

╰─% NOTMUCH_DEBUG_QUERY=y notmuch search 'subject:"not"'
Query string is:
subject:"not"
notmuch search: A Xapian exception occurred
A Xapian exception occurred parsing query: Syntax: <expression> NOT <expression>
Query string was: subject:"not"

Interestingly, putting space around the operator seems to be a
workaround. Something about turning on phrase parsing maybe?

╰─% NOTMUCH_DEBUG_QUERY=y notmuch count 'subject:" not "'
Query string is:
subject:" not "
Exclude query is:
Query((((Kspam OR Kdeleted) OR Kmuted) OR Kbad-address))
Final query is:
Query(((Tmail AND 0 * XSUBJECTnot@1) AND_NOT (((Kspam OR Kdeleted) OR Kmuted) OR Kbad-address)))
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