gitk: Add the equivalent of diff --color-words

gitk: Add the equivalent of diff --color-words

Use the newly added 'diff --word-diff=porcelain' to teach gitk a
color-words mode, with two different modes analogous to the
--word-diff=plain and --word-diff=color settings.  These are selected
by a dropdown box.

As an extra twist, automatically enable this word-diff support when
the user mentions a word-diff related option on the command line.
These options were previously ignored because they would break diff
parsing.

Both of these features are only enabled if we have a version of git
that supports --word-diff=porcelain, meaning at least 1.7.2.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Paul Mackerras <paulus@samba.org>