format-patch: "--rfc=-(WIP)" appends to produce [PATCH (WIP)]

In the previous step, the "--rfc" option of "format-patch" learned
to take an optional string value to prepend to the subject prefix,
so that --rfc=WIP can give "[WIP PATCH]".

There may be cases in which the extra string wants to come after the
subject prefix.  Extend the mechanism to allow "--rfc=-(WIP)" [*] to
signal that the extra string is to be appended instead of getting
prepended, resulting in "[PATCH (WIP)]".

In the documentation, discourage (ab)using "--rfc=-RFC" to say
"[PATCH RFC]" just to be different, when "[RFC PATCH]" is the norm.

[Footnote]

 * The syntax takes inspiration from Perl's open syntax that opens
   pipes "open fh, '|-', 'cmd'", where the dash signals "the other
   stuff comes here".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -247,6 +247,12 @@ RFC means "Request For Comments"; use this when sending
 an experimental patch for discussion rather than application.
 "--rfc=WIP" may also be a useful way to indicate that a patch
 is not complete yet ("WIP" stands for "Work In Progress").
++
+If the convention of the receiving community for a particular extra
+string is to have it _after_ the subject prefix, the string _<rfc>_
+can be prefixed with a dash ("`-`") to signal that the the rest of
+the _<rfc>_ string should be appended to the subject prefix instead,
+e.g., `--rfc='-(WIP)'` results in "PATCH (WIP)".
 
 -v <n>::
 --reroll-count=<n>::

builtin/log.c

@@ -2065,8 +2065,12 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (cover_from_description_arg)
 		cover_from_description_mode = parse_cover_from_description(cover_from_description_arg);
 
-	if (rfc && rfc[0])
+	if (rfc && rfc[0]) {
-		strbuf_insertf(&sprefix, 0, "%s ", rfc);
+		if (rfc[0] == '-')
+			strbuf_addf(&sprefix, " %s", rfc + 1);
+		else
+			strbuf_insertf(&sprefix, 0, "%s ", rfc);
+	}
 
 	if (reroll_count) {
 		strbuf_addf(&sprefix, " v%s", reroll_count);

t/t4014-format-patch.sh

@@ -1394,6 +1394,15 @@ test_expect_success '--rfc=WIP and --rfc=' '
 	test_cmp expect-raw actual
 '
 
+test_expect_success '--rfc=-(WIP) appends' '
+	cat >expect <<-\EOF &&
+	Subject: [PATCH (WIP) 1/1] header with . in it
+	EOF
+	git format-patch -n -1 --stdout --rfc="-(WIP)" >patch &&
+	grep "^Subject:" patch >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '--rfc does not overwrite prefix' '
 	cat >expect <<-\EOF &&
 	Subject: [RFC PATCH foobar 1/1] header with . in it