gitk: Show local changes properly when we have a path limit

gitk: Show local changes properly when we have a path limit

Since gitk looks for the HEAD commit to attach the fake commits for
local changes to, we can miss out on seeing the fake commits if we
have a path limit and the HEAD commit doesn't alter any of the files
in the path limit.

This fixes it by running

	git rev-list -1 $head -- $paths

if we have a path limit, and taking the result of that as the commit
to attach the fake commits to.  This means that we can be attaching
the fake commits to a different commit in each view, so we use a new
$viewmainhead($view) for that.

This also fixes a buglet where updatecommits would only fix up the
fake commits if the HEAD changed since the last call to updatecommits,
whereas it should fix them up if the HEAD has changed since this view
was last created or updated.

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