Add is:<tag> as a synonym for tag:<tag> in search terms.

I like the readability of this, it provides compatibility with people
trained in this syntax by sup, and it even saves one character.

lib/database.cc

@@ -147,6 +147,7 @@ prefix_t BOOLEAN_PREFIX_INTERNAL[] = {
 prefix_t BOOLEAN_PREFIX_EXTERNAL[] = {
     { "thread",			"G" },
     { "tag",			"K" },
+    { "is",			"K" },
     { "id",			"Q" }
 };
 

notmuch.1

@@ -373,7 +373,7 @@ terms to match against specific portions of an email, (where
 
 	attachment:<word>
 
-	tag:<tag>
+	tag:<tag> (or is:<tag>)
 
 	id:<message-id>
 
@@ -402,7 +402,7 @@ prefix can be used to search for specific filenames (or extensions) of
 attachments to email messages.
 
 For
-.BR tag: ,
+.BR tag: " and " is:
 valid tag values include
 .BR inbox " and " unread
 by default for new messages added by

notmuch.c

@@ -52,7 +52,7 @@ static const char search_terms_help[] =
     "\t\t\tto:<name-or-address>\n"
     "\t\t\tsubject:<word-or-quoted-phrase>\n"
     "\t\t\tattachment:<word>\n"
-    "\t\t\ttag:<tag>\n"
+    "\t\t\ttag:<tag> (or is:<tag>)\n"
     "\t\t\tid:<message-id>\n"
     "\t\t\tthread:<thread-id>\n"
     "\n"
@@ -66,7 +66,7 @@ static const char search_terms_help[] =
     "\t\tthe subject of an email. Quoted phrases are supported when\n"
     "\t\tsearching with: subject:\"this is a phrase\".\n"
     "\n"
-    "\t\tFor tag:, valid tag values include \"inbox\" and \"unread\"\n"
+    "\t\tFor tag: and is:, valid tag values include \"inbox\" and \"unread\"\n"
     "\t\tby default for new messages added by \"notmuch new\" as well\n"
     "\t\tas any other tag values added manually with \"notmuch tag\".\n"
     "\n"