Mailmap: Allow empty email addresses to be mapped

While it makes no sense to map some email address to an empty one, doing
things the other way around can be useful. For example when using
filter-branch with an env-filter that employs a mailmap to fix up an
import that created such broken commits with empty email addresses.
Signed-off-by: NBjörn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

mailmap.c

@@ -90,7 +90,8 @@ static void add_mapping(struct string_list *map,
 		 old_name, old_email, new_name, new_email);
 }
 
-static char *parse_name_and_email(char *buffer, char **name, char **email)
+static char *parse_name_and_email(char *buffer, char **name,
+		char **email, int allow_empty_email)
 {
 	char *left, *right, *nstart, *nend;
 	*name = *email = 0;
@@ -99,7 +100,7 @@ static char *parse_name_and_email(char *buffer, char **name, char **email)
 		return NULL;
 	if ((right = strchr(left+1, '>')) == NULL)
 		return NULL;
-	if (left+1 == right)
+	if (!allow_empty_email && (left+1 == right))
 		return NULL;
 
 	/* remove whitespace from beginning and end of name */
@@ -150,8 +151,8 @@ static int read_single_mailmap(struct string_list *map, const char *filename, ch
 			}
 			continue;
 		}
-		if ((name2 = parse_name_and_email(buffer, &name1, &email1)) != NULL)
-			parse_name_and_email(name2, &name2, &email2);
+		if ((name2 = parse_name_and_email(buffer, &name1, &email1, 0)) != NULL)
+			parse_name_and_email(name2, &name2, &email2, 1);
 
 		if (email1)
 			add_mapping(map, name1, email1, name2, email2);