Teach "guilt graph" the "-x exclude-pattern" option.

Teach "guilt graph" the "-x exclude-pattern" option.

Some projects keep a ChangeLog which every commit modifies.  This
makes the graph a very uninteresting single line of commits.  It is
sometimes useful to see how the graph would look if we ignore the
ChangeLog file.

The new -x option is useful in situations like this.  It can be
repeated several times to ignore many files.  Each argument is saved
to a temporary file and "grep -v -f $TEMPORARY" is used to filter out
the file names you want to ignore.

Also added a minimal test case and documentation.

Signed-off-by: Per Cederqvist <cederp@opera.com>
Signed-off-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>