Move the `LinkedKegs` migration into `utils.rb` so it can also be called
from `brew.rb` on startup.

os/mac: officially support Sierra.

Always run LinkedKegs migration (if needed).

test-bot: generate coverage reports on Sierra.

Ensure that `brew update` always runs the LinkedKegs migration if needed
as it may not have been run by `brew update` if it was using `--preinstall` or
a `git pull` etc.

Also, if the old paths still exist: just use them instead.

Finally, always try to unlink/unpin before link/pin.

Fixes https://github.com/Homebrew/homebrew-core/issues/4918.

We have a GM, a bunch of bottles and CI setup.

We may have two Sierra nodes available which could speed up this
relatively slow operation.

Move LinkedKegs/PinnedKegs/Locks from Library.

.codecov.yml: disable GitHub comments.

These are annoying and the status check is sufficient.

update-test: handle missing master branch.

Ensure it's always created.

Also, re-order `update-test` in `test-bot` (and `readall`) so they are
run before the test suites.

These don't make sense to be tied to the `HOMEBREW_REPOSITORY` but
instead should live in the `HOMEBREW_PREFIX` as they all relate to its
state.

Send coverage from Jenkins, don't run OS X Travis.

OS X Travis CI is particularly error-prone, slow and unnecessary for
us when we're already running (fast) OS X builds on Jenkins so use it
just for Linux testing.

Install brew-cask zsh completions by default

Warn when using Xcode 8 without CLT on 10.11

Use constants for LinkedKegs/PinnedKegs/Locks.

These definitions are scattered throughout the codebase which makes it
hard to refactor them later (my goal is to move them outside of
HOMEBREW_LIBRARY). Unify their definitions for clearer code and easier
movement later.

superenv: help Autotools with 10.12 SDK on 10.11

keg: add ALL_TOP_LEVEL_DIRECTORIES constant.

The GNU Autotools tests for whether a given symbol is defined are
reliably coming to incorrect conclusions on 10.11 with the 10.12 SDK
in Xcode 8. This overrides its decisions by forcing the right answer
in superenv using ac_cv_func_* environment variables and setting them to
"no" on 10.11. The list of problematic symbols is from

  grep 'weak$os10.11' MacOSX.sdk/usr/lib/system/libsystem_c.tbd

utils: link_src_dst_dirs: paths not files.

This is used in diagnostic (and will be used in other places later)
rather than hard-coding other directories later.

This can ask for you to remove directories (e.g.
`/usr/local/share/homebrew`) so don’t call this files to avoid
confusion.

Make scons use stdenv only if immediate build dep

Documentation Updates.

Teach brew to recognize gcc-HEAD as valid compiler