show: Avoid inadvertently closing stdout

show: Avoid inadvertently closing stdout

GMime has a nasty habit of taking ownership by default of any FILE*
handed to it va g_mime_stream_file_new. Specifically it will close the
FILE* when the stream is destroyed---even though GMime didn't open the
file itself.

To avoid this bad behavior, we have to carefully set_owner(FALSE)
after calling g_mime_stream_file_new. In the format_part_content_text
function, since commit d92146d3a6809f8ad940302af49cd99a0820665e we've
been calling g_mime_stream_file_new unconditionally, but only calling
g_mime_stream_file_set_owner(FALSE) conditionally.

This led to the FILE* being closed early when notmuch show output was
redirected to a file.

Fixing this fixes the test-suite cases that broke with the previous
commit, (which added redirected "notmuch show" calls to the test suite
to expose this bug).

Edited-by: Carl Worth <cworth@cworth.org> with a new commit message to
explain the bug and fix.