[jdk8u-ri] RFR: 8210283: Support git as an SCM alternative in the build

[Erik Joelsson]

This is a backport of the changes needed to support building from Git repos. I've tried to keep the changes minimal. The patch differs quite a bit from the original jdk8u backport of this change, mainly in the parts that only really serve to interact with Oracle custom build logic, and likely aren't used by anyone in jdk8u main. For example, I chose to keep the existing .hgtip files instead of trying to rename them to something else (.gittip or .src-rev), as updating the external dependencies we have on these file names would be too much work for this release.

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Commit messages:
 - Git support

Changes: https://git.openjdk.java.net/jdk8u-ri/pull/1/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk8u-ri&pr=1&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8210283
  Stats: 79 lines in 5 files changed: 74 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk8u-ri/pull/1.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk8u-ri pull/1/head:pull/1

PR: https://git.openjdk.java.net/jdk8u-ri/pull/1