Merge tag 'upstream/0.26.0+dfsg.1'

Upstream version 0.26.0+dfsg.1

* tag 'upstream/0.26.0+dfsg.1': (367 commits)
  DFSG changes
  CHANGELOG: document git_filter_init and GIT_FILTER_INIT
  Adding git_filter_init for initializing `git_filter` struct + unit test
  adding GIT_FILTER_VERSION to GIT_FILTER_INIT as part of convention
  tests: odb: add more low-level backend tests
  tests: odb: implement `exists_prefix` for the fake backend
  tests: odb: use correct OID length
  tests: odb: have the fake backend detect ambiguous prefixes
  tests: core: test initialization of `git_proxy_options`
  tests: network: add missing include for `git_repository_new`
  cmake: disable optimization on debug builds
  cmake: set "-D_DEBUG" on non-Windows platforms
  cmake: remove stale comment on precompiled headers
  travis: replace use of deprecated homebrew/dupes tap
  travis: install openssl explicitly
  remote: ensure we can create an anon remote on inmemory repo
  repository: don't fail to create config option in inmemory repo
  repository_item_path: return ENOTFOUND when appropriate
  tests: odb: add tests with multiple backends
  tests: odb: allow passing fake objects to the fake backend
  ...