hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions