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.

--- a/tests/test-help.t	Wed Aug 15 22:24:38 2018 +0900
+++ b/tests/test-help.t	Wed Aug 15 22:24:50 2018 +0900
@@ -1913,8 +1913,12 @@
       ----------------------------------
       1.   --tool         o      o
       2.   HGMERGE        o      o
-      3.   merge-patterns o      x
-      4.   ui.merge       x      x
+      3.   merge-patterns o (*)  x (*)
+      4.   ui.merge       x (*)  x (*)
+  
+      If "merge.strict-capability-check" configuration is true, Mercurial checks
+      capabilities of internal merge tools strictly in (*) cases above. It is
+      false by default for backward compatibility.
   
       Note:
          After selecting a merge program, Mercurial will by default attempt to