RFR: JDK-8210283: Support git as an SCM alternative in the build

[Kim Barrett]

> On Sep 4, 2018, at 6:00 PM, Erik Joelsson <erik.joelsson at oracle.com> wrote:
> 
> There are several instances of OpenJDK source being hosted in Git repositories instead of Mercurial. AdoptOpenJDK is one and some engineers use local conversions for their day to day work. Project Skara aims to investigate Git as well. To better support this, the build needs to be able to support Git as an alternative to Mercurial when creating the identification hash in the release file. It would also be convenient with a .gitignore file mirroring the .hgignore file.
> 
> This patch adds such support as well as a .gitignore file.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210283
> 
> Webrev: http://cr.openjdk.java.net/~erikj/8210283/webrev.01/index.html
> 
> /Erik

[Just a comment, not a review (I don’t feel qualified to really review this change).]

The comment starting at line 36 of make/SourceRevision.gmk needs updating.