filemerge: raise InMemoryMergeConflictsError if we hit merge conflicts in IMM

Merge conflicts might be supported in the future, but for now are kept out of
scope.

Any places where we used to call `flushall()` should be replaced with some kind
of exception. At this point, IMM M1 is no longer supported.

Differential Revision: https://phab.mercurial-scm.org/D1212

mercurial/filemerge.py

@@ -241,6 +241,12 b' def _iprompt(repo, mynode, orig, fcd, fc'
     ui = repo.ui
     fd = fcd.path()
 
+    # Avoid prompting during an in-memory merge since it doesn't support merge
+    # conflicts.
+    if fcd.changectx().isinmemory():
+        raise error.InMemoryMergeConflictsError('in-memory merge does not '
+                                                'support file conflicts')
+
     prompts = partextras(labels)
     prompts['fd'] = fd
     try:
@@ -465,11 +471,10 b' def _idump(repo, mynode, orig, fcd, fco,'
     a = _workingpath(repo, fcd)
     fd = fcd.path()
 
-    # Run ``flushall()`` to make any missing folders the following wwrite
-    # calls might be depending on.
     from . import context
     if isinstance(fcd, context.overlayworkingfilectx):
-        fcd.changectx().flushall()
+        raise error.InMemoryMergeConflictsError('in-memory merge does not '
+                                                'support the :dump tool.')
 
     util.writefile(a + ".local", fcd.decodeddata())
     repo.wwrite(fd + ".other", fco.data(), fco.flags())
@@ -688,10 +693,10 b' def _filemerge(premerge, repo, wctx, myn'
         onfailure = _("merging %s failed!\n")
         precheck = None
 
-        # If using deferred writes, must flush any deferred contents if running
-        # an external merge tool since it has arbitrary access to the working
-        # copy.
-        wctx.flushall()
+        if wctx.isinmemory():
+            raise error.InMemoryMergeConflictsError('in-memory merge does not '
+                                                    'support external merge '
+                                                    'tools')
 
     toolconf = tool, toolpath, binary, symlink
 
@@ -710,6 +715,10 b' def _filemerge(premerge, repo, wctx, myn'
     if precheck and not precheck(repo, mynode, orig, fcd, fco, fca,
                                  toolconf):
         if onfailure:
+            if wctx.isinmemory():
+                raise error.InMemoryMergeConflictsError('in-memory merge does '
+                                                        'not support merge '
+                                                        'conflicts')
             ui.warn(onfailure % fd)
         return True, 1, False
 
@@ -736,6 +745,10 b' def _filemerge(premerge, repo, wctx, myn'
 
         if r:
             if onfailure:
+                if wctx.isinmemory():
+                    raise error.InMemoryMergeConflictsError('in-memory merge '
+                                                            'does not support '
+                                                            'merge conflicts')
                 ui.warn(onfailure % fd)
             _onfilemergefailure(ui)