gitk: Allow view to specify arbitrary arguments to git-rev-list

The list of arguments to git-rev-list, including arguments that
select the range of commits, is now a part of the view specification.
If any arguments are given to gitk, they become part of the
"Command line" view, and the non-file arguments become the default
for any new views created.

Getting an error from git-rev-list is no longer fatal; instead the
error window pops up, and when you press OK, the main window just
shows "No commits selected".

The git-rev-list arguments are entered in an entry widget in the
view editor window using shell quoting conventions, not Tcl quoting
conventions.

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