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.