Fix merge failure handling in tg export

There exist two problems that show that up to now the error handling was
completely untested:

  - tg.sh uses `set -e`, so `git merge $branch; retmerge=$?` obviously
    doesn't do the right thing.
  - in recurse_deps stdin is redirected from $_depsfile.  Starting a
    shell with that doesn't give the user any chance to do something.

Reported-by: "Bernhard R. Link" <brlink@debian.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>