Prompt for passwords from the console in jgit command line tools

If we are on a Java 6 JVM we should have the Console class available,
unless the user has redirected /dev/null to stdin.  When there is a
console present we would prefer to use that for command line prompts
as that is what the user expects from a command line tool.

Change-Id: Ibaf87bb5540371d94d96d1b7e94ca002f752e5bd
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>