cmdutil: use '[committemplate]' section like as map file for style definition
Before this patch, each template definitions for 'changeset*' in
'[committemplate]' section have to be written fully from scratch,
even though many parts of them may be common.
This patch uses '[committemplate]' section like as the map file for
the style definition. All items other than 'changeset' can be referred
from others.
This can reduce total cost of template customization in
'[committemplate]' section.
When the commit template other than '[committemplate] changeset'
is chosen by 'editform', putting '[committemplate] changeset'
value into the cache of the templater causes unexpected result,
because the templater stores the specified (= chosen) template
definition into own cache as 'changeset' at construction time.
This is the reason why '[committemplate] changeset' can't be referred
from others.