tests/test-sparse-requirement.t

lfs: explicitly add the Content-Length header when uploading blobs, for py3

This was the reason for test-lfs-test-server.t#git-server complaining about an
"invalid byte in chunk length".  For some reason if this isn't explicitly added,
py3.7.1 is adding `transfer-encoding: chunked` as well as `Content-length: x`.
Wireshark flagged this as malformed.  However, if this is set, it doesn't bother
with `transfer-encoding`.

Before this patch with py3:

  PUT /objects/31cf46fbc4ecd458a0943c5b4881f1f5a6dd36c53d6167d5b69ac45149b38e5b HTTP/1.1
  Accept-Encoding: identity
  Content-length: 12
  accept: application/vnd.git-lfs
  content-type: application/octet-stream
  host: localhost:20062
  transfer-encoding: chunked
  user-agent: git-lfs/2.3.4 (Mercurial 4.9rc0+149-7eb7637e34bf)

Before this patch with py27:

  PUT /objects/31cf46fbc4ecd458a0943c5b4881f1f5a6dd36c53d6167d5b69ac45149b38e5b HTTP/1.1
  Accept-Encoding: identity
  accept: application/vnd.git-lfs
  content-type: application/octet-stream
  content-length: 12
  host: localhost:20062
  user-agent: git-lfs/2.3.4 (Mercurial 4.9rc0+149-7eb7637e34bf+20190128)

With this patch and py3, the content is the same as the py27 example.  RFC2616
says to ignore `Content-Length` if `Transfer-Encoding` is present, so maybe
there's nothing to do in the hg-server side (though I'm not sure which it is
using if presented both).

Maybe chunked encoding is better to do?  If someone knows how to suppress the
`Content-Length`, we can try that instead.

  $ hg init repo
  $ cd repo

  $ touch a.html b.html c.py d.py

  $ cat > frontend.sparse << EOF
  > [include]
  > *.html
  > EOF

  $ hg -q commit -A -m initial

  $ echo 1 > a.html
  $ echo 1 > c.py
  $ hg commit -m 'commit 1'

Enable sparse profile

  $ cat .hg/requires
  dotencode
  fncache
  generaldelta
  revlogv1
  sparserevlog
  store
  testonly-simplestore (reposimplestore !)

  $ hg debugsparse --config extensions.sparse= --enable-profile frontend.sparse
  $ ls
  a.html
  b.html

Requirement for sparse added when sparse is enabled

  $ cat .hg/requires
  dotencode
  exp-sparse
  fncache
  generaldelta
  revlogv1
  sparserevlog
  store
  testonly-simplestore (reposimplestore !)

Client without sparse enabled reacts properly

  $ hg files
  abort: repository is using sparse feature but sparse is not enabled; enable the "sparse" extensions to access!
  [255]

Requirement for sparse is removed when sparse is disabled

  $ hg debugsparse --reset --config extensions.sparse=

  $ cat .hg/requires
  dotencode
  fncache
  generaldelta
  revlogv1
  sparserevlog
  store
  testonly-simplestore (reposimplestore !)

And client without sparse can access

  $ hg files
  a.html
  b.html
  c.py
  d.py
  frontend.sparse