Changes are intensionally minimal.  Subsequent commits might refine further

I am luke warm about this change -- just wanted to compartmentalize
"forces" to _push instead of spreading logic around all helper functions

… (to be more to the point of the effect)

Closes datalad#4603

=== Do not change lines below ===
{
 "chain": [],
 "cmd": "git-sedi datatransfer checkdatapresent",
 "exit": 0,
 "extra_inputs": [],
 "inputs": [],
 "outputs": [
  "datalad/core/distributed/push.py",
  "datalad/core/distributed/tests/test_push.py"
 ],
 "pwd": "."
}
^^^ Do not change lines above ^^^

…just enable all forces - orthogonal)

=== Do not change lines below ===
{
 "chain": [],
 "cmd": "git-sedi pushall all",
 "exit": 0,
 "extra_inputs": [],
 "inputs": [],
 "outputs": [
  "datalad/core/distributed/push.py",
  "datalad/core/distributed/tests/test_push.py"
 ],
 "pwd": "."
}
^^^ Do not change lines above ^^^

Any option is about changing "automatic decision making" IMHO. I think it is
important to mention possible effects such as absence of --force for push is
a safety measure. Default --fast  is optimization and name "checkdatapresent" is
self descriptive on the effect

…auto-if-wanted config

I was going back and forth either we need some additional "datalad.push.transfer-data"
config setting which could be set per repository to indicate what "datalad push"
should do with data.  But I remain of an idea that it would be counter-productive
to establish yet another level of data transfer behavior instead of using what is
already there provided by git-annex itself.

To maintain "do the right thing" behavior by default I had decided also to make
auto-if-wanted to be the default mode of operation -- it will transfer data if there
is no explicit "wanted" setting for the remote.

…t-annex

To ease possible troubleshooting later on whenever some expected data transfer
does not happen.  Also tuned up a comment to say "copy" not "move", which is a
valid (but not supported ATM) operation.

=== Do not change lines below ===
{
 "chain": [],
 "cmd": "sed -i -e 's,transfer_data,data,g' -e 's,--transfer-data,--data,g' {outputs}",
 "exit": 0,
 "extra_inputs": [],
 "inputs": [],
 "outputs": [
  "datalad/core/distributed/push.py",
  "datalad/core/distributed/tests/test_push.py"
 ],
 "pwd": "."
}
^^^ Do not change lines above ^^^

Also placed list of constraints to a new line to make it fit 80 chars

Adapt the test posted at dataladgh-4541.  Aside from tweaking the way to
specify "use --auto", add a block that tests that the following two
behave the same when a file does not match a remote's preferred
content:

  datalad push --to=r --data=auto FILE
  git annex copy --to=r --auto FILE

Re: datalad#4620 (comment)

We already do this elsewhere in test_push.py, because running the
post-update hooks fails.  Example:
https://ci.appveyor.com/project/mih/datalad/builds/33609947/job/8ylv5wpq7l2fn93p#L1061