This rewrite attempts to sort out where the Prefix, Cellar, and
Repository are relative to the real and symlinked 'brew' command.
Also included is a --config option which dumps all of these variables.
Any top-level script must define a "BREW_FILE" that gives the path
to brew as it exists in the path. 'brew' itself just uses __FILE__ and
install.rb does a `which brew` (there may be a better way?)
The Prefix is always relative to the location of brew as it exists in
the path. Thus, whether or not /usr/local/bin/brew is a symlink or real
file, the Prefix is always /usr/local. If you have brew in some other
prefix, such as /nonstandard/bin/brew, then '/nonstandard/ will be
managed by brew instead.
The Repository, Cellar, and "Library/Homebrew" required code is always
found relative to the "real" path or brew. If brew is a real file in
/usr/local/bin/brew, then everything else will be found in /usr/local
and we'll expect a /usr/local/.git
Otherwise, we dereference brew's symlink and look for everything else
relative to that path instead.
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