gitk: Reorganize processing of arguments for git log

gitk: Reorganize processing of arguments for git log

This moves the scanning of the argument list for each view into a
new function, parseviewargs, which is called from start_rev_list.
This also makes the date mode and the merge mode be per-view rather
than global.  In merge mode, we work out the list of relevant files
in a new function called from start_rev_list, so it will be updated
on File->Reload.  Plus we now do that after running the argscmd, so
if we have one and it generates a -d or --merge option they will be
correctly handled now.

The other thing this does is to make errors detected in start_rev_list
not be fatal.  Now instead of doing exit 1 we just pop up and error
window and put "No commits selected" in the graph pane.

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