HPCC-9984 Compile direct from a version control system

First stab at a vc hook for git repository support

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-9984 Compile direct from a version control system

Don't allow workunit to set the hook - would be some security concerns.

Define an environment variable for the current thread index, in case anyone
wants it for an svn plugin.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-9984 Compile direct from a version control system

Control whether local WU can override the branch

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>