src/_update_src_copy: don't copy deleted files

Fix that _update_src_copy.sh is failing with the following error on git
repos with deleted but not yet commited files:

  updating src_copy: /home/lynxis/.cache/osmo-ttcn3-testenv/podman/make4-asan-debian-trixie/src_copy/libosmo-netif/
  rsync: [sender] link_stat "/home/lynxis/projects/osmocom/repos/libosmo-netif/doc/twrtp/README" failed: No such file or directory (2)
  rsync: [sender] link_stat "/home/lynxis/projects/osmocom/repos/libosmo-netif/doc/twrtp/twrtp-guide.t" failed: No such file or directory (2)
  rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1338) [sender=3.4.1]

I have implemented this with "comm" to substract the list of deleted
files from the list of relevant files to copy, as there was no option
for "git ls-files" to do this in one step, and as using "--exclude-from"
with this file didn't work in combination with the already passed
allowlist with neither "git ls-files --exclude-from" nor "rsync
--exclude-from".

Fixes: OS#6889
Change-Id: I232263bc5bd26c90f1e73b115480269165a1e1f8
---
M src/_update_src_copy.sh
A tests/test_update_src_copy.py
2 files changed, 94 insertions(+), 4 deletions(-)