[jdk11u-ri] RFR: 8210008: custom extension for make/SourceRevision.gmk

[Erik Joelsson]

Backporting this to 11.0.0.1 RI release to facilitate a clean backport of [JDK-8210283](https://bugs.openjdk.org/browse/JDK-8210283) which among other things adapts the source revision build logic for Git. The patch applies cleanly and only touches the source revision build logic.

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Depends on: https://git.openjdk.org/jdk11u-ri/pull/1

Commit messages:
 - JDK-8210008

Changes: https://git.openjdk.org/jdk11u-ri/pull/3/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-ri&pr=3&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8210008
  Stats: 26 lines in 1 file changed: 14 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk11u-ri/pull/3.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-ri pull/3/head:pull/3

PR: https://git.openjdk.org/jdk11u-ri/pull/3