[8u] RFR: 8210283: Support git as an SCM alternative in the build

[Severin Gehwolf]

Hi,

Please review this adaptation of the corresponding JDK 11 patch. The
JDK 11u patch didn't apply because the build system is widely different
between these two releases.

The main difference is make/common/MakeBase.gmk (JDK 8) vs
make/SourceRevision.gmk (JDK 11). I've basically rewritten those parts
of the patch. All the SCM handling in JDK 8 is in MakeBase.
FindAllReposAbs isn't present in JDK 8u.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210283
webrev: https://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210283/01/webrev/

Testing: "make --trace source-tips" on mercurial tree as well as
         the git mirror. $IMAGE_DIR/release file contains the SHA of
         the sources it was built from with 'git:' or 'hg:' prefixes.

Thoughts?

Thanks,
Severin