hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
header = header-raw.tmpl
footer = ""
changeset = changeset-raw.tmpl
difflineplus = "#line#"
difflineminus = "#line#"
difflineat = "#line#"
diffline = "#line#"
changesetparent = "# parent: #node#"
changesetchild = "# child: #node#"
filenodelink = ""
filerevision = filerevision-raw.tmpl
fileline = "#line#"
diffblock = "#lines#"
filediff = filediff-raw.tmpl
fileannotate = fileannotate-raw.tmpl
annotateline = "#author#@#rev#: #line#"