RFR: 8286105: SourceRevision.gmk should respect GIT variable

[Yasumasa Suenaga]

We can specify `git` binary via `GIT` in configure script, but it does not affect in SourceRevision.gmk .

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Commit messages:
 - 8286105: SourceRevision.gmk should respect GIT variable

Changes: https://git.openjdk.java.net/jdk/pull/8526/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8526&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286105
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8526/head:pull/8526

PR: https://git.openjdk.java.net/jdk/pull/8526