pkt-line: add options argument to read_packetized_to_strbuf()

pkt-line: add options argument to read_packetized_to_strbuf()

Update the calling sequence of `read_packetized_to_strbuf()` to take
an options argument and not assume a fixed set of options.  Update the
only existing caller accordingly to explicitly pass the
formerly-assumed flags.

The `read_packetized_to_strbuf()` function calls `packet_read()` with
a fixed set of assumed options (`PACKET_READ_GENTLE_ON_EOF`).  This
assumption has been fine for the single existing caller
`apply_multi_file_filter()` in `convert.c`.

In a later commit we would like to add other callers to
`read_packetized_to_strbuf()` that need a different set of options.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>