configitems: add a default value for "merge-tools.xxx.regappend" When trying to figure out how `hg help -v` took the Set interpolation path in f09bc2ed9100, I turned on devel warnings and noticed this (unrelated) warning: devel-warn: specifying a mismatched default value for a registered config item: 'merge-tools.beyondcompare4.regappend' '' at: c:\Users\Matt\hg\mercurial\filemerge.py:46 (_toolstr) The previous default value for this config was `None`, but that slightly complicates the code at the only site it is used, referenced above.