Fixes #671.

This is less than ideal but it gets the time on my machine down from ~6s
to ~2s when checking no taps. It still shows that we're doing way more
in `update.sh` than we need to be doing but that's a future PR.

If a requirement is for a dependent that's already installed and that
dependency is not using a `default_formula` (which would have already
been converted from a `Requirement` to `Dependency` at this stage) then
we want to stop it killing the build.

Read the discussion in https://github.com/Homebrew/homebrew-core/pull/3703

. If you
have a better idea, please file a competing PR. I'm sick to death of discussion.

Closes #662.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes #626.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Additionally include our bootstrap code in `brew.sh`, Bash utilities in
`utils.sh` and `utils/*.sh`, `superenv` shims, and the Bash completion.

Closes #654.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

e.g. Caskroom/homebrew-cask.

Thanks to UniqMartin for the fix.

Fixes #655.

Doesn't feel like a good reason to abort installation for users if the
formula author has specified a conflict that doesn't exist. Instead,
behave more like the `TapFormulaUnavailableError` but print a message
telling people to report to the formula authors.

Closes https://github.com/Homebrew/homebrew-versions/issues/1327

This reverts commit 24f7e671.

Closes #648.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Closes #584.

Signed-off-by: Xu Cheng <xucheng@me.com>

Introduce `--fetch-HEAD` option. Without this option upgrade and
outdated never fetch latest upstream commit to detect if HEAD is
outdated -- tabs are used instead. However, if option is passed,
we fetch commit from upstream, which is more time consuming,
but we can be sure that version is up-to-date or outdated.