tar archive frontend for fast-import.

tar archive frontend for fast-import.

This is an example fast-import frontend, in less than 100 lines
of Perl.  It accepts one or more tar archives on the command line,
passes them through gzcat/bzcat/zcat if necessary, parses out the
individual file headers and feeds all contained data to fast-import.
No temporary files are involved.

Each tar is treated as one commit, with the commit timestamp coming
from the oldest file modification date found within the tar.

Each tar is also tagged with an annotated tag, using the basename
of the tar file as the name of the tag.

Currently symbolic links and hard links are not handled by the
importer.  The file checksums are also not verified.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>