Make --batch -J run multiple jobs at the same time, so big batch operations
can potentially run faster.

Currently, it does not, but it still accepts the combination of options.

Hmm, probably most users of this will use --json too, and then the output
for at least some commands already contains enough information so they can
keep different async jobs straight:

	(echo foo; echo bar) | git-annex add --batch --json
	{"command":"add","success":true,"key":"SHA256E-s30--ce0c29e173009d77fa8803fae163b7c85920248add208bc378d465cae3087962","file":"foo"}
	{"command":"add","success":true,"key":"SHA256E-s30--ce0c29e173009d77fa8803fae163b7c85920248add208bc378d465cae3087962","file":"bar"}

Other commands not though; addurl does not include the url in its --json
output (though it could be made to without breaking back-compat).

Also the filename in --json output may get normalized to something
other than the input. Eg, git-annex add, for "./foo" outputs "foo".
(Due to relPathCwdToFile in batchStart, which is there to handle
absolute filepaths. So even if that case were changed, it would need to
normalize absolute filepaths.)

Maybe the json output should include a field that mirrors the input that
resulted in this line, be it a filename or an url, or a directory, or
whatever.
	
	{"command":"add","input":["./foo"],"success":true,"key":"SHA256E-s30--ce0c29e173009d77fa8803fae163b7c85920248add208bc378d465cae3087962","file":"foo"}

> This is now implemented on the batchasync branch, for all commands
> that support json output.

(Note that, --batch currently does not operate on directories.
Because of the one line or reply per line of input rule.
Adding such a field would go a long way toward allowing that, 
if it were wanted later.)

And some --batch commands like lookupkey do not support --json 
(though could be made to).

Some subcommands in --batch mode ouput a blank line instead of
json when they don't need to do anything to handle the thing they were told
to act on (eg, getting a file that is already present). (copy, drop, get,
move, add, find, whereis). That's a bit of a problem since when it's async
there would be no indication what thing they skipped,
except for their other output about the things they didn't skip.

> I looked at datalad's handling of this as an example, and it currently
> ignores the blank line, not caring when nothing needs to be done.
> Which seems about what you'd expect.. The only real reason the blank
> line is there currently is so a program can write one batch line
> and read one batch line in reply and keep things straight.
> 
> Would any program really care if git-annex didn't need to get a file
> that was already present or skipped over a file that other options
> told it not to operate on? Probably not. So, can probably get away with
> leaving the blank line as-is.

---

Making the changes to --json output to address the above stuff, if
possible, feels cleaner than wrapping input and output with a job id like
the ASYNC extension does. Those changes would also generally be an
improvement to json output in non-async mode.

> Ok, the input field is implemented, and it seems we can probably
> ignore the complication of the blank line for skipped input.
> So, it should be possible to actually implement the concurrent
> batch mode now. --[[Joey]]

[[!tag projects/datalad]]

> [[implemented|done]]. 

> See <https://git-annex/bramchable.com/tips/using_git-annex_from_your_program/>
> for some docs. --[[Joey]]
