Bash completions: speed up Tap completions

[maxim-belkin]

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• Use the HOMEBREW_REPOSITORY variable when it is set.
• Change taps="${taps} ${dir}" to taps+=" ${dir}", as the latter is significantly faster.

[BrewTestBot]

Review period will end on 2023-02-08 at 11:06:21 UTC.

[MikeMcQuaid]

Makes sense! Holding off merging until we land 4.0.0.

[BrewTestBot]

Review period skipped due to critical label.