am: --rebasing

The new option --rebasing is used internally for rebase to tell am that
it is being used for its purpose.  This would leave .dotest/rebasing to
help "completion" scripts tell if the ongoing operation is am or rebase.

Also the option at the same time stands for --binary, -3 and -k which
are always given when rebase drives am as its backend.

Using the information "am" leaves, git-completion.bash tells ongoing
rebase and am apart.

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

contrib/completion/git-completion.bash

@@ -70,7 +70,15 @@ __git_ps1 ()
 		local b
 		if [ -d "$g/../.dotest" ]
 		then
-			r="|AM/REBASE"
+			if test -f "$g/../.dotest/rebasing"
+			then
+				r="|REBASE"
+			elif test -f "$g/../.dotest/applying"
+			then
+				r="|AM"
+			else
+				r="|AM/REBASE"
+			fi
 			b="$(git symbolic-ref HEAD 2>/dev/null)"
 		elif [ -f "$g/.dotest-merge/interactive" ]
 		then