look for Brewfile in HOMEBREW_USER_CONFIG_HOME

[rrotter]

Indirectly support XDG_CONFIG_HOME by leaning on brew's support for it.

when --global is set, we'll get the brewfile from:

• $HOMEBREW_BUNDLE_FILE_GLOBAL if set
• $XDG_CONFIG_HOME/homebrew/Brewfile (if XDG_CONFIG_HOME is set and file exists)
• $HOME/.homebrew/Brewfile (if XDG_CONFIG_HOME is not set, and file exists)
• ~/.Brewfile (unconditional fallback, but if this doesn't exist either, we should probably use $HOMEBREW_USER_CONFIG_HOME/Brewfile as the new default [edit: per feedback])

Starting this as a draft because:

• looking for feedback on whether this change is welcome at all (I suspect so, per: Global Brewfile would be better placed somewhere under ~/.config or ~/.local as per XDG #1002 (comment))
• looking for feedback on what the fallback default should be if no file exists
• I still need to add tests. I'll need to add fixtures for a home dir, since currently tests are using the user's real $HOME, which we don't want to read or write from in rspec.
• need to update readme and brew bundle --help text

[MikeMcQuaid]

• $HOME/.homebrew/Brewfile (if XDG_CONFIG_HOME is not set, and file exists)
• but if this doesn't exist either, we should probably use $HOMEBREW_USER_CONFIG_HOME/Brewfile as the new default

This is the only part I'm 👎🏻 on, I don't see any users wanting this.

• looking for feedback on what the fallback default should be if no file exists

The current location.

[MikeMcQuaid]

Code mostly makes sense so far. I'd be 👍🏻 on supporting behaviour change if XDG_CONFIG_HOME is set and otherwise I'd like to leave everything as-is. Thanks!

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[rrotter]

Updated help text, added spec.

[MikeMcQuaid]

Thanks @rrotter, great work!

[rrotter]

@MikeMcQuaid, thanks. I went through several iterations on the help text before committing this. Not at all bothered that you decided to tweak it, but I just want to point out that I specifically avoided mentioning $HOMEBREW_USER_CONFIG_HOME in the help text (even though it's in the PR title) because it's a brew internal variable which can't be set by the user:

https://github.com/Homebrew/brew/blob/9fd2b27a353726b2ea1641b9b0477d5cc25ac595/bin/brew#L172-L177

[MikeMcQuaid]

Thanks @rrotter, my bad. Fixing in #1579