--- a/mercurial/commands.py	Wed Sep 30 21:22:31 2015 -0700
+++ b/mercurial/commands.py	Fri Oct 02 13:00:47 2015 -0700
@@ -2476,6 +2476,71 @@
     '''backwards compatibility with old bash completion scripts (DEPRECATED)'''
     debugnamecomplete(ui, repo, *args)
 
+@command('debugmergestate', [], '')
+def debugmergestate(ui, repo, *args):
+    """print merge state
+
+    Use --verbose to print out information about whether v1 or v2 merge state
+    was chosen."""
+    def printrecords(version):
+        ui.write(('* version %s records\n') % version)
+        if version == 1:
+            records = v1records
+        else:
+            records = v2records
+
+        for rtype, record in records:
+            # pretty print some record types
+            if rtype == 'L':
+                ui.write(('local: %s\n') % record)
+            elif rtype == 'O':
+                ui.write(('other: %s\n') % record)
+            elif rtype == 'F':
+                r = record.split('\0')
+                f, state, hash, lfile, afile, anode, ofile = r[0:7]
+                if version == 1:
+                    onode = 'not stored in v1 format'
+                    flags = r[7]
+                else:
+                    onode, flags = r[7:9]
+                ui.write(('file: %s (state "%s", hash %s)\n')
+                         % (f, state, hash))
+                ui.write(('  local path: %s (flags "%s")\n') % (lfile, flags))
+                ui.write(('  ancestor path: %s (node %s)\n') % (afile, anode))
+                ui.write(('  other path: %s (node %s)\n') % (ofile, onode))
+            else:
+                ui.write(('unrecognized entry: %s\t%s\n')
+                         % (rtype, record.replace('\0', '\t')))
+
+    ms = mergemod.mergestate(repo)
+
+    # sort so that reasonable information is on top
+    v1records = ms._readrecordsv1()
+    v2records = ms._readrecordsv2()
+    order = 'LO'
+    def key(r):
+        idx = order.find(r[0])
+        if idx == -1:
+            return (1, r[1])
+        else:
+            return (0, idx)
+    v1records.sort(key=key)
+    v2records.sort(key=key)
+
+    if not v1records and not v2records:
+        ui.write(('no merge state found\n'))
+    elif not v2records:
+        ui.note(('no version 2 merge state\n'))
+        printrecords(1)
+    elif ms._v1v2match(v1records, v2records):
+        ui.note(('v1 and v2 states match: using v2\n'))
+        printrecords(2)
+    else:
+        ui.note(('v1 and v2 states mismatch: using v1\n'))
+        printrecords(1)
+        if ui.verbose:
+            printrecords(2)
+
 @command('debugnamecomplete', [], _('NAME...'))
 def debugnamecomplete(ui, repo, *args):
     '''complete "names" - tags, open branch names, bookmark names'''

--- a/tests/test-backout.t	Wed Sep 30 21:22:31 2015 -0700
+++ b/tests/test-backout.t	Fri Oct 02 13:00:47 2015 -0700
@@ -593,6 +593,23 @@
   use 'hg resolve' to retry unresolved file merges
   [1]
   $ hg status
+  $ hg debugmergestate
+  * version 2 records
+  local: b71750c4b0fdf719734971e3ef90dbeab5919a2d
+  other: a30dd8addae3ce71b8667868478542bc417439e6
+  file: foo (state "u", hash 0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33)
+    local path: foo (flags "")
+    ancestor path: foo (node f89532f44c247a0e993d63e3a734dd781ab04708)
+    other path: foo (node f50039b486d6fa1a90ae51778388cad161f425ee)
+  $ mv .hg/merge/state2 .hg/merge/state2-moved
+  $ hg debugmergestate
+  * version 1 records
+  local: b71750c4b0fdf719734971e3ef90dbeab5919a2d
+  file: foo (state "u", hash 0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33)
+    local path: foo (flags "")
+    ancestor path: foo (node f89532f44c247a0e993d63e3a734dd781ab04708)
+    other path: foo (node not stored in v1 format)
+  $ mv .hg/merge/state2-moved .hg/merge/state2
   $ hg resolve -l  # still unresolved
   U foo
   $ hg summary

--- a/tests/test-completion.t	Wed Sep 30 21:22:31 2015 -0700
+++ b/tests/test-completion.t	Fri Oct 02 13:00:47 2015 -0700
@@ -91,6 +91,7 @@
   debugknown
   debuglabelcomplete
   debuglocks
+  debugmergestate
   debugnamecomplete
   debugobsolete
   debugpathcomplete
@@ -251,6 +252,7 @@
   debugknown: 
   debuglabelcomplete: 
   debuglocks: force-lock, force-wlock
+  debugmergestate: 
   debugnamecomplete: 
   debugobsolete: flags, record-parents, rev, date, user
   debugpathcomplete: full, normal, added, removed

--- a/tests/test-help.t	Wed Sep 30 21:22:31 2015 -0700
+++ b/tests/test-help.t	Fri Oct 02 13:00:47 2015 -0700
@@ -810,6 +810,8 @@
    debuginstall  test Mercurial installation
    debugknown    test whether node ids are known to a repo
    debuglocks    show or modify state of locks
+   debugmergestate
+                 print merge state
    debugnamecomplete
                  complete "names" - tags, open branch names, bookmark names
    debugobsolete