log: prepare log/log-tree to reuse the diffopt.close_file attribute

log: prepare log/log-tree to reuse the diffopt.close_file attribute

We are about to teach the log-tree machinery to reuse the diffopt.file
field to output to a file stream other than stdout, in line with the
diff machinery already writing to diffopt.file.

However, we might want to write something after the diff in
log_tree_commit() (e.g. with the --show-linear-break option), therefore
we must not let the diff machinery close the file (as per
diffopt.close_file.

This means that log_tree_commit() itself must override the
diffopt.close_file flag and close the file, and if log_tree_commit() is
called in a loop, the caller is responsible to do the same.

Note: format-patch has an `--output-directory` option. Due to the fact
that format-patch's options are parsed first, and that the parse-options
machinery accepts uniquely abbreviated options, the diff options
`--output` (and `-o`) are shadowed. Therefore close_file is not set to 1
so that cmd_format_patch() does *not* need to handle the close_file flag
differently, even if it calls log_tree_commit() in a loop.

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