commit-graph: don't pass filename to load_commit_graph_one_fd_st()

commit-graph: don't pass filename to load_commit_graph_one_fd_st()

An earlier change implemented load_commit_graph_one_fd_st() in a way
that was bug-compatible with earlier code in terms of the "graph file
%s is too small" error message printing out the path to the
commit-graph (".git/objects/info/commit-graph").

But change that, because:

 * A function that takes an already-open file descriptor also needing
   the filename isn't very intuitive.

 * The vast majority of errors we might emit when loading the graph
   come from parse_commit_graph(), which doesn't report the
   filename. Let's not do that either in this case for consistency.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>