am: officially deprecate -b/--binary option

We have had these options as harmless no-op for more than 3 years without
officially deprecating them.  Let's announce the deprecation and start
warning against their use, but without failing the command just not yet,
so that we can later repurpose the option if we want to in the future.

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

Documentation/RelNotes/1.7.10.txt

@@ -26,8 +26,8 @@ Compatibility Notes
    Git v1.7.8 or newer.
 
  * The "--binary/-b" options to "git am" have been a no-op for quite a
-   while and was deprecated in mid 2008 (v1.6.0).  When you give these
-   options to "git am", it will now fail with an error.
+   while and were deprecated in mid 2008 (v1.6.0).  When you give these
+   options to "git am", it will now warn and ask you not to use them.
 
 
 Updates since v1.7.9

git-am.sh

@@ -380,8 +380,8 @@ do
 	-i|--interactive)
 		interactive=t ;;
 	-b|--binary)
-		echo >&2 "The -b/--binary option was deprecated in 1.6.0 and removed in 1.7.10."
-		die "Please adjust your scripts."
+		echo >&2 "The $1 option has been a no-op for long time, and"
+		echo >&2 "it will be removed. Please do not use it anymore."
 		;;
 	-3|--3way)
 		threeway=t ;;