GitHub has requested we request our usage of shallow clones. As a
result:

- do default to shallow clones on CI
- do not do --shallow-since when setting up new repositories in
  `brew update`
- do not support shallow clones of homebrew-core at all

We may consider in future unshallowing all clones unconditionally.

Co-authored-by: Maxim Belkin <maxim.belkin@gmail.com>

This should help with cases like
https://github.com/Homebrew/homebrew-bundle/issues/751
and is good practise in general.

Also, document the `brew update --preinstall` flag that is being used
here so others can run it manually e.g. as part of CI if needed.

Fix regression from https://github.com/Homebrew/brew/pull/7260.

This code makes me sad.

I tested the workaround with the newest Git and the Catalina system Git
and it wasn't needed in either case. We can re-add it if we get
complaints with supported Git versions.

Fixes https://github.com/Homebrew/brew/issues/7259

This is needed for `brew test-bot` when no global Git configuration is
present.

- Output a message whenever a non-default remote is set.
- Ensure that `CoreTap.instance.install` uses the right remote.

This variable allows a user to configure how often `brew update` runs.
With the previous implementation, however, it was still pretty slow as
it would run some of `brew update` even within the timeout. This feels
related to complaints about Homebrew "feeling slow" since we enabled
this. Address this by:

- making `HOMEBREW_AUTO_UPDATE_SECS` behave more like
  `HOMEBREW_NO_AUTO_UPDATE` and entirely skip the invocation of
  `brew update` if the relevant tap (homebrew-core or homebrew-cask
  depending on command invoked) has been checked in the last
  `HOMEBREW_AUTO_UPDATE_SECS`
- Only require/check the update of a single tap rather than every tap
- Increase the default value of `HOMEBREW_AUTO_UPDATE_SECS` from 1m
  to 5m to cut people a bit more slack. We're not updating things
  often enough for 1m not to feel a bit overkill and 5m feels
  appropriate for a Homebrew "session" to only require a single update.

Use active voice when applicable, better verbs & adjectives, preposition usage, proper tense, etc.

Document short options and common global options. Apply the same indent
as other command help strings.

This would allow the Linuxbrew core tap to live in the Homebrew
organisation.

We removed a few old, deprecated repos recently and people with them
still tapped who get confused at the `brew update` error message.

`--quiet` doesn't seem to be working as expected.

- Add some `odeprecated`
- Make some `odeprecated` now `odisabled`
- Remove `odisabled` code.
- Remove old update migrations
- Remove GCC 4.0 compiler
- Remove Tiger-only code
- Remove 32-bit-only code
- Remove use of LD64
- Remove GCC 4.3 - 4.8 support.

Older systems will install Homebrew via git://, but can switch to https:// if a newer curl is installed.

This serves a similar purpose to the HOMEBREW_FORCE_HOMEBREW_ORG
variable but applies to more settings.

Fixes #4867

No need to check for both variables, because they're always set together.
Also, this harmonizes with the CURL equivalent just above.

Because of this messing with the user's path:
https://github.com/Homebrew/brew/blob/
    efc02899/bin/brew#L68
brew uses /usr/bin/git over brewed git, even when the former is
problematically old.
There may also be other reasons a user prefers to use brewed git.

There was already a HOMEBREW_FORCE_BREWED_CURL option and a
HOMEBREW_SYSTEM_CURL_TOO_OLD check to set it. This mostly copies those
to implement HOMEBREW_FORCE_BREWED_GIT & HOMEBREW_SYSTEM_GIT_TOO_OLD.

See also: https://github.com/Linuxbrew/brew/issues/736