RFR: update .gitignore

[Xin Wang]

update .gitignore

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Commit messages:
 - update .gitignore

Changes: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/4/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmh-jdk-microbenchmarks&pr=4&range=00
  Stats: 27 lines in 1 file changed: 22 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh-jdk-microbenchmarks pull/4/head:pull/4

PR: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/4