chgserver: include [extdiff] in confighash

extdiff's uisetup will register new commands. If we do not include it in
confighash, changes to [extdiff] will not get new commands registered.
This patch adds extdiff to confighash and makes it possible for chg to pass
test-extdiff.t.

hgext/chgserver.py

@@ -77,7 +77,10 b' def _hashlist(items):'
     return util.sha1(str(items)).hexdigest()
 
 # sensitive config sections affecting confighash
-_configsections = ['extensions']
+_configsections = [
+    'extdiff',  # uisetup will register new commands
+    'extensions',
+]
 
 # sensitive environment variables affecting confighash
 _envre = re.compile(r'''\A(?: