pretty: provide human date format

Add the placeholders %ah and %ch to format author date and committer
date, like --date=human does, which provides more humanity date output.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/pretty.c b/pretty.c
index e5b33ba..b1ecd03 100644
--- a/pretty.c
+++ b/pretty.c
@@ -745,6 +745,9 @@ static size_t format_person_part(struct strbuf *sb, char part,
 	case 'I':	/* date, ISO 8601 strict */
 		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(ISO8601_STRICT)));
 		return placeholder_len;
+	case 'h':	/* date, human */
+		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(HUMAN)));
+		return placeholder_len;
 	case 's':
 		strbuf_addstr(sb, show_ident_date(&s, DATE_MODE(SHORT)));
 		return placeholder_len;