Protect get_author_ident_from_commit() from filenames in work tree

Protect get_author_ident_from_commit() from filenames in work tree

We used to use "cat-file commit $commit" to extract the original
author information from existing commit, but an earlier commit
5ac2715 (Consistent message encoding while reusing log from an
existing commit) changed it to use "git show -s $commit".  If
you have a file in your work tree that can be interpreted as a
valid object name (e.g. "HEAD"), this conversion will not work.

Disambiguate by marking the end of revision parameter on the
comand line with an explicit "--" to fix this.

This breakage is most visible with rebase when a file called
"HEAD" exists in the worktree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>