Patch rebased on latest master attached.  The new test implements a
small proxy for the behavior exhibited by git and its libexec scripts.

Call for help: the test does not pass!  I get this error:

  hello: run.c:284: exec_in_user_namespace: Unexpected error: No such
file or directory.

Could someone more familiar with user namespaces, etc help me work
this out?

`~Eric