let commands that show changesets use templates.
mechanism is same as hgweb templates.
old show_changeset code is still used for now if no template given,
because it is faster than template code when verbose or debug.
simple template can be given on command line using -t, --template.
example:
hg log -t '{author|person}\n'
complex template can be put in template map file, given on command line
using --map-file.
we give two example map files:
map-log.compact prints 3 lines of output for every change.
map-log.verbose prints exact same output as default "hg log -v".
map files are searched where user says, then in template path as backup.
example:
hg log --map-file map-log.compact
defaults can be set in hgrc with ui.logtemplate and ui.logmap.