trailer: process trailers from input message and arguments

Implement the logic to process trailers from the input message
and from arguments.

At the beginning trailers from the input message are in their
own "in_tok" doubly linked list, and trailers from arguments
are in their own "arg_tok" doubly linked list.

The lists are traversed and when an "arg_tok" should be "applied",
it is removed from its list and inserted into the "in_tok" list.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: read and process config information

Read the configuration to get trailer information, and then process
it and store it in a doubly linked list.

The config information is stored in the list whose first item is
pointed to by:

static struct trailer_item *first_conf_item;

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: process command line trailer arguments

Parse the trailer command line arguments and put
the result into an arg_tok doubly linked list.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: parse trailers from file or stdin

Read trailers from a file or from stdin, parse the trailers and then
put the result into a doubly linked list.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: put all the processing together and print

This patch adds the process_trailers() function that
calls all the previously added processing functions
and then prints the results on the standard output.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: add interpret-trailers command

This patch adds the "git interpret-trailers" command.
This command uses the previously added process_trailers()
function in trailer.c.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: add tests for "git interpret-trailers"

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: execute command from 'trailer.<name>.command'

Let the user specify a command that will give on its standard output
the value to use for the specified trailer.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
trailer: add tests for commands in config file

And add a few other tests for some special cases.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Documentation: add documentation for 'git interpret-trailers'

While at it add git-interpret-trailers to "command-list.txt".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>