Merge branch 'ps/3.0-remote-deprecation'

Following the procedure we established to introduce breaking changes for Git 3.0, allow an early opt-in for removing support of $GIT_DIR/branches/ and $GIT_DIR/remotes/ directories to configure remotes. * ps/3.0-remote-deprecation: remote: announce removal of "branches/" and "remotes/" builtin/pack-redundant: remove subcommand with breaking changes ci: repurpose "linux-gcc" job for deprecations ci: merge linux-gcc-default into linux-gcc Makefile: wire up build option for deprecated features