web: Made elapsed time calculation dynamic (javascript). This allow safe caching of the pages by the browser and still display the right amount of elapsed time upon page refresh. If javascript is disabled, absolute time is displayed, leaving it readable. All the templates have been updated.

extdiff: add repository root as a variable Some external diff tools (notably Plan 9 diff(1)) require the absolute path to the file being diffed for proper function. A root variable was added to inform an external tool of the repository root (the tool is invoked with the cwd set to tmproot).

help: move hgignore man page into built-in help (issue2769)