blame: silently ignore invalid ignore file objects

blame: silently ignore invalid ignore file objects

Since 610e2b9240 (blame: validate and peel the object names on the
ignore list, 2020-09-24) git blame reports checks if objects specified
with --ignore-rev and in files loaded with --ignore-revs-file and config
option blame.ignoreRevsFile are actual objects and dies if they aren't.
The intent is to report typos to the user.

This also breaks the ability to use a single ignore file for multiple
repositories.  Typos are presumably less likely in files than on the
command line, so alerting is less useful here.  Restore that feature by
skipping non-commits without dying.

Reported-by: Jean-Yves Avenard <jyavenard@mozilla.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Reviewed-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>