ConfigMerge should make sure the base and new configuration file paths are different

[vladak]

The ConfigMerge tool should make sure that (canonicalized) paths for the base and new files are different to avoid nasty surprises.

Inspired by #4133 (reply in thread)_

[vladak]

Also, the Docker scripts should check the return value of merge_config_files() and react accordingly, i.e. report the error and continue:

opengrok/docker/start.py

Lines 519 to 520 in c53c7ab

	merge_config_files(read_only_config_file, OPENGROK_CONFIG_FILE,
	tmp_out, jar=OPENGROK_JAR, loglevel=log_level)