notmuch show: limit display to only matching messages

notmuch show: limit display to only matching messages

This patch changes the default behaviour of notmuch show to display only
messages that match the search expression.  However, --entire-thread
option is provided to display all messages in threads that matched the
search expression.

It is deemed that will be more useful for human users on the command line.
Scripts can be modified to include the --entire-thread option so that they
can display all messages once more.

Example:

$ notmuch search subject:git AND thread:23d99d0f364f93e90e15df8b42eddb5b
thread:23d99d0f364f93e90e15df8b42eddb5b      July 31 [4/12] Johan Herland; [RFCv2 00/12] Foreign VCS helper program for CVS repositories (inbox unread)

Note that in this thread 4 out of 12 messages matched.  The default show
behaviour is to show only those messages that match:

$ notmuch show subject:git AND thread:23d99d0f364f93e90e15df8b42eddb5b | grep 'message{' | wc -l
4

With the --entire-thread option the output will contain all dozen
messages:

$ notmuch show --entire-thread subject:git AND thread:23d99d0f364f93e90e15df8b42eddb5b | grep 'message{' | wc -l
12

Signed-off-by: Bart Trojanowski <bart@jukie.net>