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

[Erik Joelsson]

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