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.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program