filemerge: when not keeping premerge, don't write markers to context

When premerge is enabled (as it is for non-binary inputs by default)
and the markers are not kept, we currently still write it to the
output context and then restore the previous content right after. With
the refactoring in the previous patch, we can easily avoid that step
and instead write the output in the opposite case (i.e. when it's
successful or when the markers are supposed to be kept).

Differential Revision: https://phab.mercurial-scm.org/D12149

mercurial/filemerge.py

@@ -411,7 +411,7 b' def _verifytext(input, ui):'
         raise error.Abort(msg)
 
 
-def _premerge(repo, local, other, base, toolconf, backup):
+def _premerge(repo, local, other, base, toolconf):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     if symlink or local.fctx.isabsent() or other.fctx.isabsent():
         return 1
@@ -445,15 +445,13 b' def _premerge(repo, local, other, base, '
         merged_text, conflicts = simplemerge.simplemerge(
             ui, local, base, other, mode=mode
         )
-        # fcd.flags() already has the merged flags (done in
-        # mergestate.resolve())
-        local.fctx.write(merged_text, local.fctx.flags())
+        if not conflicts or premerge in validkeep:
+            # fcd.flags() already has the merged flags (done in
+            # mergestate.resolve())
+            local.fctx.write(merged_text, local.fctx.flags())
         if not conflicts:
             ui.debug(b" premerge successful\n")
             return 0
-        if premerge not in validkeep:
-            # restore from backup and try again
-            _restorebackup(local.fctx, backup)
     return 1  # continue merging
 
 
@@ -879,12 +877,6 b' def partextras(labels):'
     }
 
 
-def _restorebackup(fcd, backup):
-    # TODO: Add a workingfilectx.write(otherfilectx) path so we can use
-    # util.copy here instead.
-    fcd.write(backup.data(), fcd.flags())
-
-
 def _makebackup(repo, ui, wctx, fcd):
     """Makes and returns a filectx-like object for ``fcd``'s backup file.
 
@@ -1123,7 +1115,6 b' def filemerge(repo, wctx, mynode, orig, '
                 other,
                 base,
                 toolconf,
-                backup,
             )
             # we're done if premerge was successful (r is 0)
             if not r: