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.

mercurial/configitems.py

@@ -1786,6 +1786,13 b' coreconfigitem('
 )
 coreconfigitem(
     b'merge-tools',
+    br'.*\.regappend$',
+    default=b"",
+    generic=True,
+    priority=-1,
+)
+coreconfigitem(
+    b'merge-tools',
     br'.*\.symlink$',
     default=False,
     generic=True,

mercurial/filemerge.py

@@ -158,7 +158,7 b' def findexternaltool(ui, tool):'
             continue
         p = util.lookupreg(k, _toolstr(ui, tool, b"regname"))
         if p:
-            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend", b""))
+            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend"))
             if p:
                 return p
     exe = _toolstr(ui, tool, b"executable", tool)