obshistory: make obslog work when a commit doesn't have any description This logic is taken from logcmdutil.changesetprinter class. It checks that commit description is not empty before extracting the first line (before this patch it would try to index an empty list). Stripping description also makes obslog behave more like regular log when there are trailing/leading whitespaces.

obshistory: use a more appropriate type for obslog entries cand is actually a regular changeset and not 'M' (graphmod.MISSINGPARENT). And 'M' is only used for graph edges, but cand is drawn as a graph node. Visually there's no effect, but the code makes more sense this way.