Otherwise random e.g. `pr-123` tags may cause `brew update` to update
to the wrong version.

Previously, .simplecov called `SimpleCov.result` to store the coverage
result, and ignored the return value. `SimpleCov.result`'s return can be
slow to calculate, which wastes a lot of time when it's ignored.

This commit extracts the code needed to store the SimpleCov result from
`SimpleCov.result`, and calls it directly, without doing the busywork to
compute the return value every time.

In my testing, this more than halves the time taken to run all the
integration tests.

If we have a HOMEBREW_REPOSITORY and HOMEBREW_PREFIX mismatch (now the
default) then we can block access to the whole of HOMEBREW_REPOSITORY
rather than just the HOMEBREW_LIBRARY and `.git`.

This reverts commit a124680b.

Will need to be smarter than this, since people can't force Travis to
update. I'm losing count of the amount of times Travis has forced a change
of plans around Homebrew this year.

Closes https://github.com/Homebrew/brew/issues/1096.

`sha1` isn't a very accurate description of what this value can be
because it can be any git commit reference. For example, `brew test-bot`
uses `brew update-test --commit=HEAD`.

This hack has been in Homebrew Cask for more than two years
(since 51f93e6dc9c3da4ab2118459ea95e45c104386ec), and it originated even
earlier (6d2f7bc55af0b2aa915b2396d213e30a4446256c).

Cask tests apparently aren't even run on Travis anymore,
so this can be safely removed.

Signed-off-by: Bob W. Hogg <rwhogg@linux.com>

This adds a CMake cache entry to std_cmake_args specifying that the
function clock_gettime is not available on 10.11 in order to avoid
runtime errors such as

  dyld: lazy symbol binding failed: Symbol not found: _clock_gettime

when the build system is confused by Xcode 8's weak symbols.

Other weak symbols on 10.11, which may merit the same treatment in the
future, can be found with

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

It’ll only get printed for people getting updated to tags now and these
are people who haven’t run a `dev-cmd` so we want to air on the side of
telling them less stuff that will confuse them and assume people who
have manually made another `git` branch will know how to get back to it.

For tagged commits produces the output:
- `1.0.1`

For untagged commits with a dirty tree produces the output:
- `1.0.1-19-g23efbc5-dirty`

Performance:
```
git describe --tags --dirty 2> /dev/null
0.07s user 0.01s system 96% cpu 0.086 total
```

This means we can tag any commit without needing to manually remember
to bump the revision every time.