Merge branch 'jk/config-cleanup'

Code clean-up around use of configuration variables.

* jk/config-cleanup:
  sequencer: simplify away extra git_config_string() call
  gpg-interface: drop pointless config_error_nonbool() checks
  push: drop confusing configset/callback redundancy
  config: use git_config_string() for core.checkRoundTripEncoding
  diff: give more detailed messages for bogus diff.* config
  config: use config_error_nonbool() instead of custom messages
  imap-send: don't use git_die_config() inside callback
  git_xmerge_config(): prefer error() to die()
  config: reject bogus values for core.checkstat

imap-send.c

@@ -1346,7 +1346,7 @@ static int git_imap_config(const char *var, const char *val,
 		server.port = git_config_int(var, val, ctx->kvi);
 	else if (!strcmp("imap.host", var)) {
 		if (!val) {
-			git_die_config("imap.host", "Missing value for 'imap.host'");
+			return config_error_nonbool(var);
 		} else {
 			if (starts_with(val, "imap:"))
 				val += 5;