GetArchive: honouring the path parameter

In the GitLab application, this is used for download
links in "subtrees" of the repository.

An archival metadata file is not expected (nor is it on
the Git side).

I've checked that `git archive` behaves similarly, i.e., just
an inclusion rule, with no re-anchoring of the files at the top
of the archive.
4 jobs for topic/default/GetArchive in 2 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Main
passed #112799
tests-current

00:00:35

0.0%
 
  Compat
passed #112800
install

00:00:21

passed #112802
tests-hg-default

00:01:35

0.0%
passed #112801
tests-hg-stable

00:01:30

0.0%