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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).