diff -r f4277a35c42c -r 183df3df6031 mercurial/commands.py
--- a/mercurial/commands.py	Fri Jan 04 16:04:48 2019 +0100
+++ b/mercurial/commands.py	Wed Nov 07 15:45:09 2018 -0800
@@ -4935,8 +4935,7 @@
 
             if mark:
                 if markcheck:
-                    with repo.wvfs(f) as fobj:
-                        fdata = fobj.read()
+                    fdata = repo.wvfs.tryread(f)
                     if filemerge.hasconflictmarkers(fdata) and \
                         ms[f] != mergemod.MERGE_RECORD_RESOLVED:
                         hasconflictmarkers.append(f)