net/url: strip relative path components for JoinPath

[cuishuang]

Fixes #54385

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Message from xie cui:

Patch Set 1:

(1 comment)

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Message from Damien Neil:

Patch Set 1:

(4 comments)

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This PR (HEAD: 0efc271) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/422715 to see it.

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Message from shuang cui:

Patch Set 4:

(3 comments)

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Message from shuang cui:

Patch Set 4:

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Message from Damien Neil:

Patch Set 4:

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Message from shuang cui:

Patch Set 4:

(1 comment)

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Message from Damien Neil:

Patch Set 4:

(1 comment)

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