RFR [15] 8238918 idea.sh should work with both mercurial and git repo

[Roger Riggs]

Please review a small change to the idea.sh script that generates an 
IntelliJ project.
It detects either Mercurial or Git repos and generates the appropriate 
vcs.xml.

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-idea-git-8238918/
Issue:
  https://bugs.openjdk.java.net/browse/JDK-8238918

Thanks, Roger