info: avoid broken GitHub URL for removed formulae

[bayandin]

When a formula has been removed from its tap but is still installed (e.g. terraform, which was disabled on 2025-04-04 and later removed from homebrew-core), FromKegLoader loads it from the keg's .brew/<name>.rb with the tap still set from the install receipt. github_info then built a "From:" URL by joining the tap's GitHub remote with formula.path.relative_path_from(tap.path), producing a path full of ../ components:

Before:

$ brew info terraform
...
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/../../../../opt/terraform/.brew/terraform.rb

After:

$ brew info terraform
...
From: /opt/homebrew/opt/terraform/.brew/terraform.rb

The fix falls back to the formula's local path when its file is not inside its tap's directory, matching the existing behaviour for formulae with no tap (or no remote). The "#{tap.path}/" (with trailing /) containment check is the same idiom already used in dev-cmd/pr-pull.rb and cask/artifact/symlinked.rb.

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Cursor (Claude Opus 4.7) was used to investigate the root cause, propose the fix, and draft the tests under my direction and review. I verified end-to-end that brew info terraform now prints the local path (and that brew info git still prints the correct GitHub URL for a formula present in its tap), and ran brew style, brew typecheck, and brew tests --only=cmd/info locally.

[MikeMcQuaid]

Makes sense, thanks!