shortlog: introduce --email-only to only show emails

When a shortlog caller wants to group output by, say, author email, they
can easily express this with:

    $ git shortlog --group=format:%ae

and restrict output to specific email(s) with the new `--group-filter`
option introduced by the previous commit.

But they are not able to apply the same treatment to identities that
appear in trailers. Doing:

    $ git shortlog -e --group=format:%ae --group=trailer:Co-authored-by

will produce funky results, interspersing proper emails with full "Name
<email>" identities from the Co-authored-by trailer (or anything else
that might appear there), like:

    461  [email protected]
     11  Taylor Blau <[email protected]>

So if the caller wants to restrict output to a set of matching email
addresses (say, "[email protected]"), they cannot do it with a
`--group-filter`, since it would discard the group "Taylor Blau
<[email protected]>".

Introduce a new `--email-only` option, which extracts the email
component of an identity from all shortlog groups, including trailers.
It behaves similarly to the `-e` option, but replaces its output with
just the email component, instead of adding it on to the end.

Now, `shortlog` callers can perform:

    $ git shortlog -s --group=author --group=trailer:Co-authored-by \
        --email-only --group-filter="<[email protected]>"
       472  <[email protected]>

to obtain the output they want.

Signed-off-by: Taylor Blau <[email protected]>

Documentation/git-shortlog.txt

@@ -39,6 +39,11 @@ OPTIONS
 --email::
 	Show the email address of each author.
 
+--email-only::
+	Show only the email address of each author, or committer. If
+	using a different kind of group (e.g. trailers, custom format,
+	etc.), only values which contain name/email-pairs will be shown.
+
 --format[=<format>]::
 	Instead of the commit subject, use some other information to
 	describe each commit.  '<format>' can be any string accepted

builtin/shortlog.c

@@ -108,9 +108,13 @@ static int parse_ident(struct shortlog *log,
 	maillen = ident.mail_end - ident.mail_begin;
 
 	map_user(&log->mailmap, &mailbuf, &maillen, &namebuf, &namelen);
-	strbuf_add(out, namebuf, namelen);
-	if (log->email)
-		strbuf_addf(out, " <%.*s>", (int)maillen, mailbuf);
+	if (log->email_only) {
+		strbuf_addf(out, "<%.*s>", (int)maillen, mailbuf);
+	} else {
+		strbuf_add(out, namebuf, namelen);
+		if (log->email)
+			strbuf_addf(out, " <%.*s>", (int)maillen, mailbuf);
+	}
 
 	return 0;
 }
@@ -198,6 +202,8 @@ static void insert_records_from_trailers(struct shortlog *log,
 		strbuf_reset(&ident);
 		if (!parse_ident(log, &ident, value))
 			value = ident.buf;
+		else if (log->email_only)
+			continue;
 
 		if (!strset_add(dups, value))
 			continue;
@@ -370,12 +376,19 @@ void shortlog_init(struct shortlog *log)
 
 void shortlog_finish_setup(struct shortlog *log)
 {
-	if (log->groups & SHORTLOG_GROUP_AUTHOR)
-		string_list_append(&log->format,
-				   log->email ? "%aN <%aE>" : "%aN");
-	if (log->groups & SHORTLOG_GROUP_COMMITTER)
-		string_list_append(&log->format,
-				   log->email ? "%cN <%cE>" : "%cN");
+	if (log->email_only) {
+		if (log->groups & SHORTLOG_GROUP_AUTHOR)
+			string_list_append(&log->format, "<%aE>");
+		if (log->groups & SHORTLOG_GROUP_COMMITTER)
+			string_list_append(&log->format, "<%cE>");
+	} else {
+		if (log->groups & SHORTLOG_GROUP_AUTHOR)
+			string_list_append(&log->format,
+					   log->email ? "%aN <%aE>" : "%aN");
+		if (log->groups & SHORTLOG_GROUP_COMMITTER)
+			string_list_append(&log->format,
+					   log->email ? "%cN <%cE>" : "%cN");
+	}
 
 	string_list_sort(&log->trailers);
 	string_list_sort(&log->group_filter);
@@ -397,6 +410,8 @@ int cmd_shortlog(int argc, const char **argv, const char *prefix)
 			 N_("suppress commit descriptions, only provides commit count")),
 		OPT_BOOL('e', "email", &log.email,
 			 N_("show the email address of each author")),
+		OPT_BOOL(0, "email-only", &log.email_only,
+			 N_("only show the email address of each author")),
 		OPT_CALLBACK_F('w', NULL, &log, N_("<w>[,<i1>[,<i2>]]"),
 			N_("linewrap output"), PARSE_OPT_OPTARG,
 			&parse_wrap_args),

shortlog.h

@@ -30,6 +30,7 @@ struct shortlog {
 	struct string_list format;
 
 	int email;
+	int email_only;
 	struct string_list mailmap;
 	FILE *file;
 };

t/t4201-shortlog.sh

@@ -400,6 +400,34 @@ test_expect_success '--no-group-filter reset group filters' '
 	test_cmp expect actual
 '
 
+test_expect_success '--email-only shows emails from author' '
+	cat >expect <<-\EOF &&
+	     2	<[email protected]>
+	EOF
+	git shortlog -ns --group=author --email-only -2 HEAD >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '--email-only shows emails from committer' '
+	cat >expect <<-\EOF &&
+	     2	<[email protected]>
+	EOF
+	git shortlog -ns --group=committer --email-only -2 HEAD >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '--email-only shows emails from trailers with idents' '
+	cat >expect <<-\EOF &&
+	     1	<[email protected]>
+	     1	<[email protected]>
+	EOF
+	# at this point, HEAD~3 has a trailer "Repeated-trailer: Foo",
+	# which is not shown here since it cannot be parsed as an ident
+	git shortlog -ns --group=trailer:some-trailer --email-only -3 \
+		HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'shortlog can match multiple format groups' '
 	GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME" \
 		git commit --allow-empty -m "identical names" &&