--strip option for tg export

This introduces the '--strip[=N]' option for 'tg export --quilt' to enable
export of a flat quilt series, even if the tg branch names are structured
hierarchically.

It may be useful if you - like me - structure your topgit branches like
"t/<NN>/<NAME>" to keep the overview in git, but don't want the exported
quilt series to be spread across subdirectories.

In contrast to '--flatten' this option removes the first 'N' or all
subdirectories from the topgit branch name, while keeping compatible with
the existing options. The remaining ones can still be flattened. Patch
names can still get numbered.

Signed-off-by: Heiko Hund <hhund@astaro.com>
Signed-off-by: Robin Green <greenrd@greenrd.org>