RFR: 8253034: Update symbol generation to accomodate Git as the SCM

[Joe Darcy]

The symbol generation script assumed Mercurial was being used as the SCM. This should now be updated to Git.

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Commit messages:
 - Update symbol generation to accomodate Git as the SCM

Changes: https://git.openjdk.java.net/jdk/pull/161/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=161&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253034
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/161.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/161/head:pull/161

PR: https://git.openjdk.java.net/jdk/pull/161