git annex add -u now supported, analagous to git add -u

Unlike git add -u, git annex add -u does not update the index for files
removed from the working tree. But then, "git add ." stages removals,
and "git annex add ." does not, so that's an existing divergence.

Seems that --update --batch would need to run git ls-files once per line of
batch input, which would surely be too slow, so just throw an error for
that.

This commit was supported by the NSF-funded DataLad project.

doc/todo/annex_add___40__-u__124__--update__41___mode.mdwn

@@ -1,3 +1,5 @@
 to supplement 'git add -u' behavior -- to add only updated (tracked only, no untracked) files to be committed.  ATM all files would be added, including untracked.
 
 [[!meta author=yoh]]
+
+> [[done]]

doc/todo/annex_add___40__-u__124__--update__41___mode/comment_1_bde2b1e2c45e110d56ce98b43dd77743._comment

@@ -0,0 +1,7 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2017-04-07T19:41:02Z"
+ content="""
+Good idea, adding.
+"""]]