Re: [PATCH 3/4] add -u: only show pathless 'add -u' warning when changes exist outside cwd

Jonathan Nieder [off-list ref] writes:

A common workflow in large projects is to chdir into a subdirectory of
interest and only do work there:

	cd src
	vi foo.c
	make test
	git add -u
	git commit

The upcoming change to 'git add -u' behavior would not affect such a
workflow: when the only changes present are in the current directory,
'git add -u' will add all changes, and whether that happens via an
implicit "." or implicit ":/" parameter is an unimportant
implementation detail.

The warning about use of 'git add -u' with no pathspec is annoying
because it serves no purpose in this case.  So suppress the warning
unless there are changes outside the cwd that are not being added.

That is a logical extension of the reason why we do not emit
warnings when run at the top level.  A user who has known about and
is very much accustomed to the "current directory only" behaviour
may run "git add -u/-A" always from the top in the current project
she happens to be working on until Git 2.0 happens, and will not get
any warnings.  We are already robbing the chance to learn about and
prepare for the upcoming change from her.  And this patch makes it
even more so.  It does not make things fundamentally worse, but it
makes it more likely to hurt such a user.

The implemenation appears to run an extra diff_files() in addition
to the one we already have to run in order to implement the "add -u"
to collect modified/deleted paths.  Is that the best we can do?  

I am wondering if we can special case the no-pathspec case to have
add_files_to_cache() call underlying run_diff_files() without any
pathspec, inspect the paths that are modified and/or deleted in the
update_callback, add ones that are under the $prefix while noticing
the ones outside as warning worthy events.