make add_object_array_with_context interface more sane

When you resolve a sha1, you can optionally keep any context found during the resolution, including the path and mode of a tree entry (e.g., when looking up "HEAD:subdir/file.c"). The add_object_array_with_context function lets you then attach that context to an entry in a list. Unfortunately, the interface for doing so is horrible. The object_context structure is large and most object_array users do not use it. Therefore we keep a pointer to the structure to avoid burdening other users too much. But that means when we do use it that we must allocate the struct ourselves. And the struct contains a fixed PATH_MAX-sized buffer, which makes this wholly unsuitable for any large arrays. We can observe that there is only a single user of the "with_context" variant: builtin/grep.c. And in that use case, the only element we care about is the path. We can therefore store only the path as a pointer (the context's mode field was redundant with the object_array_entry itself, and nobody actually cared about the surrounding tree). This still requires a strdup of the pathname, but at least we are only consuming the minimum amount of memory for each string. We can also handle the copying ourselves in add_object_array_*, and free it as appropriate in object_array_release_entry. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>