am: support --allow-empty to record specific empty patches

This option helps to record specific empty patches in the middle
of an am session, which does create empty commits only when:

    1. the index has not changed
    2. lacking a branch

When the index has changed, "--allow-empty" will create a non-empty
commit like passing "--continue" or "--resolved".

Signed-off-by: 徐沛文 (Aleen) <aleen42@vip.qq.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-am.txt b/Documentation/git-am.txt
index 7676bd5..09107fb 100644
--- a/Documentation/git-am.txt
+++ b/Documentation/git-am.txt
@@ -18,7 +18,7 @@
 	 [--quoted-cr=<action>]
 	 [--empty=(stop|drop|keep)]
 	 [(<mbox> | <Maildir>)...]
-'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=(diff|raw)])
+'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=(diff|raw)] | --allow-empty)
 
 DESCRIPTION
 -----------
@@ -200,6 +200,11 @@
 	the e-mail message; if `diff`, show the diff portion only.
 	Defaults to `raw`.
 
+--allow-empty::
+	After a patch failure on an input e-mail message lacking a patch,
+	create an empty commit with the contents of the e-mail message
+	as its log message.
+
 DISCUSSION
 ----------