Resolve src through realpath in install_renamed

[dariaguy]

Fixing untouched etc configs being left stale on upgrade.
After the new keg is linked, Formula#prefix returns opt_prefix, so the ascend in append_default_if_different walks via the opt/ symlink and the HOMEBREW_CELLAR guard rejects every level — the sibling-keg lookup never runs and <file>.default is always written. Resolving src through realpath first puts the walk back on the cellar path so the lookup fires and untouched configs can advance on upgrade.

Tested locally by adding a line to a conf in https://github.com/redis-developer/homebrew-core/blob/redis-add-line-to-conf/Formula/r/redis.rb, created a bottle and ran brew upgrade redis

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  Used claude, verified manually the fix

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[Copilot]

Pull request overview

This PR fixes a regression in InstallRenamed#append_default_if_different where, after a keg is opt-linked, the logic that detects “unchanged config matching an older bottled default” fails to locate sibling kegs in the Cellar, causing *.default files to be written unnecessarily and leaving untouched configs stale on upgrade.

Changes:

• Resolve the src path through realpath before ascend so the walk is based on the Cellar path rather than opt_prefix.
• Add a new spec covering the opt-linked keg upgrade scenario for Formula#install_etc_var.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/install_renamed.rb	Adjusts path resolution in append_default_if_different to make sibling-keg lookup work when the keg is opt-linked.
Library/Homebrew/test/formula_spec.rb	Adds a test reproducing the opt-linked keg scenario and asserting config replacement behavior.

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[MikeMcQuaid]

Thanks @dariaguy, looks good and appreciate you taking the time to do this!