commit-graph: return with errors during write

commit-graph: return with errors during write

The write_commit_graph() method uses die() to report failure and
exit when confronted with an unexpected condition. This use of
die() in a library function is incorrect and is now replaced by
error() statements and an int return type. Return zero on success
and a negative value on failure.

Now that we use 'goto cleanup' to jump to the terminal condition
on an error, we have new paths that could lead to uninitialized
values. New initializers are added to correct for this.

The builtins 'commit-graph', 'gc', and 'commit' call these methods,
so update them to check the return value. Test that 'git commit-graph
write' returns a proper error code when hitting a failure condition
in write_commit_graph().

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>