hgmerge: add and use more tool variables This patch adds even more customisable variables with tool paths. Also moves $FILEMERGE where it belongs.

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123