filemerge: add internal merge tool to dump files forcibly Internal merge tool :dump implies premerge. Therefore, files aren't dumped, if premerge runs successfully. This undocumented behavior might confuse users, if they want to always dump files. But just making :dump omit premerge might cause backward compatibility issue for existing automation. This patch adds new internal merge tool :forcedump, which works as same as :dump, but omits premerge always. Internal tools annotated with "nomerge" should merge "change and delete" correctly, but _forcedump() can't. Therefore, it is annotated with "mergeonly" to always omit premerge, even though it doesn't merge files actually. This patch also adds explanation about premerge to :dump, to clarify how :dump actually works. BTW, this patch specifies internal tools with "internal:" prefix in newly added test scenario in test-merge-tools.t, even though this prefix is already deprecated. This is only for similarity to other tests in test-merge-tools.t.

--- a/tests/test-help.t	Sat May 13 03:28:36 2017 +0900
+++ b/tests/test-help.t	Sat May 13 03:31:42 2017 +0900
@@ -1760,11 +1760,18 @@
         accordingly be named "a.txt.local", "a.txt.other" and "a.txt.base" and
         they will be placed in the same directory as "a.txt".
   
+        This implies permerge. Therefore, files aren't dumped, if premerge runs
+        successfully. Use :forcedump to forcibly write files out.
+  
       ":fail"
         Rather than attempting to merge files that were modified on both
         branches, it marks them as unresolved. The resolve command must be used
         to resolve these conflicts.
   
+      ":forcedump"
+        Creates three versions of the files as same as :dump, but omits
+        premerge.
+  
       ":local"
         Uses the local 'p1()' version of files as the merged version.