This fixes the ignoring in libgit2 clients that don't read nested
`.gitignore` files.

Add language method to `OS::Mac`.

diagnostic: remove duplicate checks.

Don't require/recommend ownership of /usr/local.

Set HOMEBREW_DEVELOPER automatically

Apple reset this on every OS X major (and some minor) updates and it
always proves a painful and unnecessary step. Instead just check the
directories we actually care about are writable.

This may mean if these directories do not already exist (although they
are now created by the installed) that `brew link` will fail and require
manual intervention but this seems to be superior for both new and the
majority of existing users.

Save aliases in INSTALL_RECEIPT

We’re defining developers as people who have run a dev-cmd at least
once.

Skip upgrade of pinned dependency if it's outdated

README and ISSUE_TEMPLATE: quote $(brew --repo)

dev-cmd: add `--help` to all developer commands.

docs: Explain Tab instantiation

Warn when changing branch on update

Also, flag those that we never want to be in a manpage.

This was really confusing to me, and I had to go looking through the Tab
history, where I found a comment explaining it that has since been
deleted. This wasn't a great experience.

This commit basically adds that explanation back in.

This reverts commit 2498d99a.

Forbid to install formual if it has pinned dependencies.

Prompt user to unpin pinned dependencies of the formula

Always a good idea to quote command substitution.