[foreign-jextract] RFR: adding libgit2 sample.

[Athijegannathan Sundararajan]

GitClone.java is a simple Java program to clone a repo from Github.

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Commit messages:
 - adding libgit2 sample

Changes: https://git.openjdk.java.net/panama-foreign/pull/182/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/182/webrev.00
  Stats: 126 lines in 2 files changed: 126 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/182.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/182/head:pull/182

PR: https://git.openjdk.java.net/panama-foreign/pull/182