bufio: avoid rescanning buffer multiple times in ReadSlice

[andrius4669]

When existing data in buffer does not have delimiter,
and new data is added with b.fill(), continue search from
previous point instead of starting from beginning.

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Message from Dave Cheney:

Patch Set 1: Code-Review-1

Thank you for this change. The development tree is currently frozen ahead of the 1.11 release [0]. To proceed with this change please consider the following suggestions

a. if this change is response to an issue targeted at Go 1.11, please include the text "fixes #NNN" where NNN is the issue this change relates to [1].
b. if this is a performance improvement, please include benchmark results using benchcmp [2] showing the potential improvements this change makes. Please be aware that this change may not be accepted during the Go 1.11 code freeze.

0: https://github.com/golang/go/wiki/Go-Release-Cycle

1. https://golang.org/doc/contribute.html#commit_changes
2. https://godoc.org/golang.org/x/tools/cmd/benchcmp

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Message from Andrius Lukas Narbutas:

Patch Set 2:

Patch Set 1: Code-Review-1

Thank you for this change. The development tree is currently frozen ahead of the 1.11 release [0]. To proceed with this change please consider the following suggestions

a. if this change is response to an issue targeted at Go 1.11, please include the text "fixes #NNN" where NNN is the issue this change relates to [1].
b. if this is a performance improvement, please include benchmark results using benchcmp [2] showing the potential improvements this change makes. Please be aware that this change may not be accepted during the Go 1.11 code freeze.

0: https://github.com/golang/go/wiki/Go-Release-Cycle

1. https://golang.org/doc/contribute.html#commit_changes
2. https://godoc.org/golang.org/x/tools/cmd/benchcmp

Thanks for information.
This should be performance improvement, I'll try benchmarking it.

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This PR (HEAD: 08e7d2f) has been imported to Gerrit for code review.

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Message from Gerrit User 5065:

Patch Set 4: Patch Set 3 was rebased

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Message from Gerrit User 5065:

Patch Set 4: Run-TryBot+1

I'll let Ian or Robert decide on this one.

Seems fine, but without performance numbers I can't be sure how much it's worth any additional complexity.

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Message from Gerrit User 5976:

Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=c6bac192

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Message from Gerrit User 5976:

Patch Set 4: TryBot-Result+1

TryBots are happy.

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This PR is being closed because golang.org/cl/113535 has been merged.