### Please describe the problem.

can't fetch in parallel from a host over ssh if authentication is password-based

### What steps will reproduce the problem?

try to  get -J4  from a host which has ssh authentication password-only (no key)


### What version of git-annex are you using? On what operating system?

6.20170101+gitg93d69b1-1~ndall+1
with newer version (6.20170220+gitg75a15e1ad-1~ndall+1) looks slightly different but to the same "effect"

### Please provide any additional information below.

[[!format sh """
$> git annex get -J4
get sourcedata/sub-sid000004/ses-siemens0/anat/sub-sid000004_ses-siemens0_acq-MPRAGE_run-01_T1w.dicom.tgz get sourcedata/sub-sid000004/ses-siemens0/fmap/sub-sid000004_ses-siemens0_acq-3mm_run-01_phasediff.dicom.tgz get sourcedata/sub-sid000005/ses-siemens1/func/sub-sid000005_ses-siemens1_task-life_acq-2mm692_run-04_bold.dicom.tgz get sourcedata/sub-sid000005/ses-siemens1/func/sub-sid000005_ses-siemens1_task-life_acq-2mm748_run-03_bold.dicom.tgz (transfer already in progress, or unable to take transfer lock) 
  Unable to access these remotes: origin
(from origin...) (from origin...) 
  Try making some of these repositories available:


  	2e44be07-8f1a-4c11-a7cb-464802b87b26 -- mvdoc@smaug:/mnt/btrfs/dbic/inbox/dbic-ds-3mm/dbic/pulse_sequences
   	b2ff2964-c31b-4784-b094-2bebb336da91 -- mvdoc@smaug:/mnt/btrfs/dbic/inbox/dbic-ds/dbic/pulse_sequences
   	d486ea11-98dc-42d3-9640-e5713acfb675 -- yoh@rolando:/inbox/BIDS/dbic/1000-dbic-dataset [origin]
failed
get sourcedata/sub-sid000005/ses-siemens1/func/sub-sid000005_ses-siemens1_task-life_acq-2mm754_run-05_bold.dicom.tgz (from origin...) 
(from origin...) 
...
yohtest@rolando.cns's password: yohtest@rolando.cns's password: yohtest@rolando.cns's password: yohtest@rolando.cns's password

"""]]

I have entered password just once -- didn't try to enter it multiple times into the void ;)  but I guess it would be neat if annex could handle this situation gracefully (e.g. initiate central ssh controller first before spawning parallel getters) and demand password once


[[!meta author=yoh]]
[[!tag projects/datalad]]

> [[done]]; this was fixed for ssh, except for in the case of `GIT_SSH`.
> I don't think it can be supported for `GIT_SSH`, and don't think it's
> worth leaving this open for something that can't be fixed. --[[Joey]]
