hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
abort: repository a not found!
255
requesting all changes
abort: error: Connection refused
255
abort: repository a not found!
255
abort: destination '../a' already exists
1
abort: repository a not found!
255
abort: destination 'q' already exists