Suggest 'add' in am/revert/cherry-pick.

Now that we have decided to make 'add' behave like 'update-index'
(and therefore fully classify update-index as strictly plumbing)
the am/revert/cherry-pick family of commands should not steer the
user towards update-index.  Instead send them to the command they
probably already know, 'add'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -401,14 +401,14 @@ do
 		changed="$(git-diff-index --cached --name-only HEAD)"
 		if test '' = "$changed"
 		then
-			echo "No changes - did you forget update-index?"
+			echo "No changes - did you forget to use 'git add'?"
 			stop_here_user_resolve $this
 		fi
 		unmerged=$(git-ls-files -u)
 		if test -n "$unmerged"
 		then
 			echo "You still have unmerged paths in your index"
-			echo "did you forget update-index?"
+			echo "did you forget to use 'git add'?"
 			stop_here_user_resolve $this
 		fi
 		apply_status=0

git-revert.sh

@@ -155,7 +155,7 @@ Conflicts:
 		uniq
 	    } >>"$GIT_DIR/MERGE_MSG"
 	    echo >&2 "Automatic $me failed.  After resolving the conflicts,"
-	    echo >&2 "mark the corrected paths with 'git-update-index <paths>'"
+	    echo >&2 "mark the corrected paths with 'git-add <paths>'"
 	    echo >&2 "and commit the result."
 	    case "$me" in
 	    cherry-pick)