Fix race condition

[bshafiee]

When multiple IndexWriter instances call Flush concurrently, they hit race or index out of range for sortTmp since it's getting modified from multiple go routines.
example:

goroutine 59 [running]:
github.com/meerkat/vendor/github.com/google/codesearch/index.sortPost(0xc023ab0000, 0x32f2, 0x800000)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:648 +0x298
github.com/meerkat/vendor/github.com/google/codesearch/index.(*IndexWriter).mergePost(0xc0000d62c0, 0xc001e8e300)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:291 +0x13c
github.com/meerkat/vendor/github.com/google/codesearch/index.(*IndexWriter).Flush(0xc0000d62c0)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:210 +0x1f1
github.com/meerkat/index.(*indexer).Index(0xc00011e000, 0x13a56e0, 0xc001fb4060, 0x0, 0x0)
	/Users/behrooz/go/src/github.com/meerkat/index/indexer.go:239 +0x429
main.main.func1(0xc0002c4280, 0xc001e4e6d0, 0xc00011c000, 0x30, 0x13a25a0, 0xc00011e000, 0xc00039b800)
	/Users/behrooz/go/src/github.com/meerkat/main.go:88 +0x206
created by main.main
	/Users/behrooz/go/src/github.com/meerkat/main.go:76 +0x334
panic: runtime error: index out of range

goroutine 181 [running]:
github.com/meerkat/vendor/github.com/google/codesearch/index.sortPost(0xc01fab0000, 0x3b65, 0x800000)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:628 +0x29f
github.com/meerkat/vendor/github.com/google/codesearch/index.(*IndexWriter).mergePost(0xc0011d2000, 0xc0000a6240)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:291 +0x13c
github.com/meerkat/vendor/github.com/google/codesearch/index.(*IndexWriter).Flush(0xc0011d2000)
	/Users/behrooz/go/src/github.com/meerkat/vendor/github.com/google/codesearch/index/write.go:210 +0x1f1
github.com/meerkat/index.(*indexer).Index(0xc00011e000, 0x13a56e0, 0xc0003a7ae0, 0x0, 0x0)
	/Users/behrooz/go/src/github.com/meerkat/index/indexer.go:239 +0x429
main.main.func1(0xc0002c4280, 0xc001e4e6d0, 0xc00011c000, 0x30, 0x13a25a0, 0xc00011e000, 0xc00039b500)
	/Users/behrooz/go/src/github.com/meerkat/main.go:88 +0x206
created by main.main
	/Users/behrooz/go/src/github.com/meerkat/main.go:76 +0x334

This fix simply moves global unprotected vars to local func scope. We could alternatively include them in IndexWriter struct too.

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[bshafiee]

I signed it!

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CLAs look good, thanks!

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[bshafiee]

@dgryski @rsc would you mind taking a look pretty please? it's a tiny PR.

[dgryski]

LGTM. However, I don't have a commit bit for this repo.

[bshafiee]

ping @rsc