RFR: Remove .hgignore and add .gitignore

[Eric Caspole]

Remove .hgignore and add .gitignore now we are in github.

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Commit messages:
 - Remove .hgignore and add .gitignore

Changes: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/3/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmh-jdk-microbenchmarks&pr=3&range=00
  Stats: 12 lines in 2 files changed: 7 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/3.diff
  Fetch: git fetch https://git.openjdk.java.net/jmh-jdk-microbenchmarks pull/3/head:pull/3

PR: https://git.openjdk.java.net/jmh-jdk-microbenchmarks/pull/3