[ticket/17732] Report real merge conflicts in the automatic updater - #7068

Checklist:

• Correct branch: master for new features; 3.3.x for fixes
• Tests pass
• Code follows coding guidelines: master and 3.3.x
• Commit follows commit message format

Tracker ticket:

https://tracker.phpbb.com/browse/PHPBB-17732

Since #6216 (PHPBB-16570) a file with a real merge conflict is treated as already merged and silently left out of the update. There are two causes in diff_files.php:

1. The in_array() check is inverted, so a conflict is never added to the list on a first run.
2. On a first run, merged_orig_output() (the board side of every conflict) equals the board file whenever all of phpBB's changes to that file conflict. So $empty is true and the conflict is skipped even with !in_array().

A conflicted file now only counts as merged by the user on a later run, when it is already in the conflict list. Otherwise it is added to the list.

tests/installer/diff_files_test.php runs the real diff_files task on fixtures:

• a pure conflict;
• a conflict plus a non-conflicting phpBB change;
• a conflict kept by the user on a later run;
• a clean three-way merge;
• a file already equal to the new version.

Both conflict cases fail without the fix.