hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory