RFR: 8048184 : (s) handle mercurial dev build version string

[Mike Duigou]

Hello all;

After pushing JDK-8047925 it was discovered that unofficial development builds of Mercurial use a different version string format and get_source.sh needs an enhancement to correctly parse this version string.

I have also incorporated some cleanup suggested by Dave Pointon, made the script more defensive in it's handling of numeric values and replaced the use of 'which' with the more reliable 'command -v'

jbsbug: https://bugs.openjdk.java.net/browse/JDK-8048184
webrev: http://cr.openjdk.java.net/~mduigou/JDK-8048184/0/webrev/

Mike