gitk: Use a text widget for the file list

gitk: Use a text widget for the file list

This lets us do things like highlighting all the entries for which
the corresponding part of the diff is at least partly visible in the
commit/patch display window, and in future it will let us display
the file list in a hierarchical form rather than as a flat file list.

Signed-off-by: Paul Mackerras <paulus@samba.org>