fix(runtime-core): prevent redundant updates to currentInstance

[edison1105]

close #12631

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB (+19 B)	37.9 kB (+9 B)	34.2 kB (-10 B)
vue.global.prod.js	158 kB (+19 B)	57.8 kB (+9 B)	51.5 kB (+95 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.3 kB (+19 B)	18.2 kB (+11 B)	16.6 kB (+14 B)
createApp	54.3 kB (+19 B)	21.1 kB (+8 B)	19.3 kB (+10 B)
createSSRApp	58.4 kB (+19 B)	22.9 kB (+12 B)	20.8 kB (+11 B)
defineCustomElement	59.1 kB (+19 B)	22.7 kB (+9 B)	20.7 kB (+10 B)
overall	68.3 kB (+19 B)	26.3 kB (+10 B)	23.9 kB (+22 B)

[pkg-pr-new]

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@vue/compiler-core

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@vue/compiler-dom

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@vue/compiler-ssr

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@vue/compiler-sfc

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@vue/reactivity

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@vue/runtime-core

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@vue/runtime-dom

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@vue/server-renderer

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@vue/shared

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vue

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@vue/compat

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commit: acb325c