hgmerge: add and use more tool variables
This patch adds even more customisable variables with tool paths.
Also moves $FILEMERGE where it belongs.
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<a href="http://www.selenic.com/mercurial/" title="Mercurial"><div style="float:right;">Mercurial</div></a><a href="?cmd=summary;style=gitweb">#repo|escape#</a> / file revisions
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