fastq_mergepairs: merging stats should be written to log file if --log is used

[frederic-mahe]

Current situation:

• without --log: vsearch header + merging stats are written to stderr
• with --log: vsearch header + time and memory are written to log file, and vsearch header + merging stats are written to stderr

I think it should be:

• with --log: vsearch header + merging stats + time and memory are written to log file, and vsearch header is written to stderr (use --quiet to remove if need be)

(covered` by tests in commit https://github.com/frederic-mahe/vsearch-tests/commit/2e692a4695bd2ce91c49bb4982a482b5679d4a63)

[torognes]

Should be fixed in commit 9009225.

[frederic-mahe]

thanks!