hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
transaction abort!
rollback completed
abort: impossible time zone offset: 4444444
transaction abort!
rollback completed
abort: invalid date: '1\t15.1'
transaction abort!
rollback completed
abort: invalid date: 'foo bar'
transaction abort!
rollback completed
abort: invalid date: ' 1 4444'
transaction abort!
rollback completed
abort: date exceeds 32 bits: 111111111111
abort: No such file or directory: .../test/bar