filemerge: use fctx.write() in the internal:dump tool, instead of copy

This is slower but allows this tool to work with the "deferred writes"
milestone of in-memory merge.

The performance hit is not too noticiable since this only used for the :dump
merge tool during a conflict.

Differential Revision: https://phab.mercurial-scm.org/D617

mercurial/filemerge.py

@@ -468,7 +468,7 b' def _idump(repo, mynode, orig, fcd, fco,'
     a = _workingpath(repo, fcd)
     fd = fcd.path()
 
-    util.copyfile(a, a + ".local")
+    util.writefile(a + ".local", fcd.decodeddata())
     repo.wwrite(fd + ".other", fco.data(), fco.flags())
     repo.wwrite(fd + ".base", fca.data(), fca.flags())
     return False, 1, False