Changes

src/_update_src_copy: don't copy deleted files

This script gets by used osmo-dev's Makefiles after gen_makefile.py
--autoreconf-in-src-copy (as used by testenv in osmo-ttcn3-hacks) for
running "autoreconf -fi" in a separate directory.

Fix that it fails 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