Add a tool to import external repositories

Add a tool to import external repositories

This commit adds a tool to aid with the importing of external git
repositories. import-external-git.pl allows a developer to easily
update the src/external portion of the tree to the latest release
of externally developed software. It's currently intended to support
our use of Heimdal functions, but could be extended to support any
project whose code we wish to use in the kernel, and which has a
git repo.

Change-Id: I6d91474c5de67e57fb73ff4d59c5afe9bb14c240
Reviewed-on: http://gerrit.openafs.org/1932
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>