filemerge: add config knob to check capabilities of internal merge tools
For historical reason, Mercurial assumes capabilities of internal
merge tools as below while examining rules to decide merge tool,
regardless of actual capabilities of them.
=============== ====== ========
specified via binary symlinks
=============== ====== ========
--tool o o
HGMERGE o o
merge-patterns o (*) x (*)
ui.merge x (*) x (*)
=============== ====== ========
This causes:
- unintentional internal merge tool is chosen for binary files via
merge-patterns section of configuration file
- explicit configuration of internal merge tool for symlinks is
ignored unintentionally
But on the other hand, simple "check capability strictly" might break
backward compatibility (e.g. existing merge automations), because it
changes the result of merge tool selection.
Therefore, this patch adds config knob "merge.strict-capability-check"
to control whether capabilities of internal merge tools should be
checked strictly or not.
If this configuration is true, capabilities of internal merge tools
are checked strictly in (*) cases above.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).