builtin/commit-graph.c: dereference tags in builtin

builtin/commit-graph.c: dereference tags in builtin

When given a list of commits, the commit-graph machinery calls
'lookup_commit_reference_gently()' on each element in the set and treats
the resulting set of OIDs as the base over which to close for
reachability.

In an earlier collection of commits, the 'git commit-graph write
--reachable' case made the inner-most call to
'lookup_commit_reference_gently()' by peeling references before they
were passed over to the commit-graph internals.

Do the analog for 'git commit-graph write --stdin-commits' by calling
'lookup_commit_reference_gently()' outside of the commit-graph
machinery, making the inner-most call a noop.

Since this may incur additional processing time, surround
'read_one_commit' with a progress meter to provide output to the caller.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>