merge: give hint as to how to discover uncommitted changes

Many users will try 'hg diff' here, and it will not show them missing files.

mercurial/merge.py

@@ -457,7 +457,8 def update(repo, node, branchmerge, forc
                     raise util.Abort(_("nothing to merge (use 'hg update'"
                                        " or check 'hg heads')"))
             if not force and (wc.files() or wc.deleted()):
-                raise util.Abort(_("outstanding uncommitted changes"))
+                raise util.Abort(_("outstanding uncommitted changes "
+                                   "(use 'hg status' to list changes)"))
         elif not overwrite:
             if pa == p1 or pa == p2: # linear
                 pass # all good

tests/test-merge-force.out

@@ -1,6 +1,6
 created new head
 % local deleted a file, remote removed
-abort: outstanding uncommitted changes
+abort: outstanding uncommitted changes (use 'hg status' to list changes)
 resolving manifests
 removing a
 0 files updated, 0 files merged, 1 files removed, 0 files unresolved

tests/test-merge1.out

@@ -40,7 +40,7 created new head
 Contents of b should be "this is file b1"
 This is file b1
 %% merge fails
-abort: outstanding uncommitted changes
+abort: outstanding uncommitted changes (use 'hg status' to list changes)
 %% merge expected!
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 (branch merge, don't forget to commit)
@@ -54,7 +54,7 M b
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 created new head
 %% merge of b should fail
-abort: outstanding uncommitted changes
+abort: outstanding uncommitted changes (use 'hg status' to list changes)
 %% merge of b expected
 merging for b
 merging b

tests/test-up-local-change.out

@@ -107,7 +107,7 summary: 2
 
 abort: crosses branches (use 'hg merge' or 'hg update -C' to discard changes)
 failed
-abort: outstanding uncommitted changes
+abort: outstanding uncommitted changes (use 'hg status' to list changes)
 failed
 resolving manifests
  overwrite False partial False