gitk: Limit diff display to listed paths by default

gitk: Limit diff display to listed paths by default

When the user has specified a list of paths, either on the command line
or when creating a view, gitk currently displays the diffs for all files
that a commit has modified, not just the ones that match the path list.
This is different from other git commands such as git log.  This change
makes gitk behave the same as these other git commands by default, that
is, gitk only displays the diffs for files that match the path list.

There is now a checkbox labelled "Limit diffs to listed paths" in the
Edit/Preferences pane.  If that is unchecked, gitk will display the
diffs for all files as before.

When gitk is run with the --merge flag, it will get the list of unmerged
files at startup, intersect that with the paths listed on the command line
(if any), and use that as the list of paths.

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