git_async_cat: don't mis-abort replaced process

git_async_cat: don't mis-abort replaced process

When a git process gets replaced (e.g. due to new
epochs/alternates), we must be careful and not abort the wrong
one.

I suspect this fixes the problem exacerbated by --batch-command.
It was theoretically possible w/o --batch-command, but it seems
to have made it surface more readily.

This should fix "Failed to retrieve generated blob" errors from
PublicInbox/ViewVCS.pm appearing in syslog

Link: https://public-inbox.org/meta/20230209012932.M934961@dcvr/