Merge tag 'upstream/0.21.0'

Merge tag 'upstream/0.21.0'

* tag 'upstream/0.21.0': (723 commits)
  test: remove assembla clone test
  test: use read-only account
  http: fix typo in credentials logic
  ssl: init everything all the time
  ssl: init also without threads
  ssl: cargo-cult thread safety
  ssl: use locking
  remote: update documentation
  netops: init OpenSSL once under lock
  revwalk: more sensible array handling
  pathspec: use C guards in header
  treebuilder: insert sorted
  remote: fix rename docs
  Bump version to 0.21.0
  Change SOVERSION at API breaks
  React to review feedback
  Win32: Fix object::cache::threadmania test on x64
  Win32: Fix diff::workdir::submodules test #2361
  Win32: Fix failing clone_mirror test
  remote: don't free the remote on delete
  ...