configuration.xml gets truncated in 1.12.26

[alclonky]

Describe the bug
After upgrading the docker based opengrok installation from 1.12.23 to 1.12.26 the service starts with ArrayIndexOutOfBoundsExeption.
The cause is an bad configuration.xml
The configuration.xml generated at startup starts with ... <truncated> and is incomplete and not an valid xml

Maybe introduced by #4475

To Reproduce
having to much projects/to big projects in 1.12.26?

Expected behavior
works as in 1.12.23

[vladak]

I think this boils down to using the ConfigMerge utility inside a Command wrapper:

opengrok/tools/src/main/python/opengrok_tools/config_merge.py

Line 68 in ee009d0

out_file.write(cmd.getoutputstr())

The ConfigMerge utility should support a path based option to write the output to and the tools should be adapted to use that if output to a file is desired.