templater: add "diff" template function
"diff" allows to embed changes in the target revision into template
output, even if the command itself doesn't take "--patch" option
Combination of "[committemplate]" configuration and "diff" template
function can achieve the feature like issue231 ("option to have diff
displayed in commit editor buffer")
http://bz.selenic.com/show_bug.cgi?id=231
For example, templating below can be used to add each "diff" output
lines "HG: " prefix::
{splitlines(diff) % 'HG: {line}\n'}
This patch implements "diff" not as "a template keyword" but as "a
template function" to take include/exclude patterns at runtime.
It allows to specify target files of command (by -I/-X command line
options) and "diff" separately.