Many parts of Homebrew assume that a version string beginning with
"HEAD" is, in fact, a head build. A stable version that begins with
"HEAD" violates this assumption and causes problems, as it's treated
as a head build in some places and as a stable build in others.

If it's not there, error out rather than fetching to avoid creating
versionless cache files, which cause `brew cleanup` to crash.

This smooths over the transition for users who have an existing
bundle config in this location due to having gems installed at the old
path.

Closes #2203.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Closes #2198.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Otherwise the user's specified emoji will be used and the test will
fail.

Taps can include completion scripts for external commands under
`completions/bash`, `completions/fish`, or `completions/zsh`. `brew tap`
will automatically install these into the correct directories during
install.