This fixes the way the example log program decides if a merge
commit should be shown when a pathspec is given. Also makes it
easier to use the pathspec API to just check "does a tree match
anything in the pathspec" without allocating a match list.
This adds more command line processing to the example version of
log. In particular, this adds the funky command line processing
that allows an arbitrary series of revisions followed by an
arbitrary number of paths and/or glob patterns.
The actual logging part still isn't implemented.
