graphmod: safer code when a changeset has two identical parents

While this situation should never under normal use, some real
life repos sometimes contain such changesets (older hg versions,
broken rebases, etc...)

hgweb was displaying an "Internal error" in this case, and graphlog
displayed a redundant branch all the way to null: it does not cost us
much to just ignore this extra parent when constructing the DAG.

mercurial/graphmod.py

@@ -32,7 +32,7 b' def revisions(repo, start, stop):'
     cur = start
     while cur >= stop:
         ctx = repo[cur]
-        parents = [p.rev() for p in ctx.parents() if p.rev() != nullrev]
+        parents = set([p.rev() for p in ctx.parents() if p.rev() != nullrev])
         yield (cur, CHANGESET, ctx, sorted(parents))
         cur -= 1
 
@@ -47,7 +47,7 b' def filerevs(repo, path, start, stop, li'
     count = 0
     while filerev >= 0 and rev > stop:
         fctx = repo.filectx(path, fileid=filerev)
-        parents = [f.linkrev() for f in fctx.parents() if f.path() == path]
+        parents = set([f.linkrev() for f in fctx.parents() if f.path() == path])
         rev = fctx.rev()
         if rev <= start:
             yield (rev, CHANGESET, fctx.changectx(), sorted(parents))
@@ -65,7 +65,7 b' def nodes(repo, nodes):'
     include = set(nodes)
     for node in nodes:
         ctx = repo[node]
-        parents = [p.rev() for p in ctx.parents() if p.node() in include]
+        parents = set([p.rev() for p in ctx.parents() if p.node() in include])
         yield (ctx.rev(), CHANGESET, ctx, sorted(parents))
 
 def colored(dag):

tests/test-glog.t

@@ -887,3 +887,39 b' File + limit + -ra:b, b < tip, (b - a) <'
   | |  summary:     (33) head
   | |
 
+Do not crash or produce strange graphs if history is buggy
+
+  $ commit 36 "buggy merge: identical parents" 35 35
+  $ hg glog -l5
+  @  changeset:   36:95fa8febd08a
+  |  tag:         tip
+  |  parent:      35:9159c3644c5e
+  |  parent:      35:9159c3644c5e
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:36 1970 +0000
+  |  summary:     (36) buggy merge: identical parents
+  |
+  o  changeset:   35:9159c3644c5e
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     0
+  |
+  o  changeset:   34:fea3ac5810e0
+  |  parent:      32:d06dffa21a31
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:34 1970 +0000
+  |  summary:     (34) head
+  |
+  | o  changeset:   33:68608f5145f9
+  | |  parent:      18:1aa84d96232a
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:33 1970 +0000
+  | |  summary:     (33) head
+  | |
+  o |    changeset:   32:d06dffa21a31
+  |\ \   parent:      27:886ed638191b
+  | | |  parent:      31:621d83e11f67
+  | | |  user:        test
+  | | |  date:        Thu Jan 01 00:00:32 1970 +0000
+  | | |  summary:     (32) expand
+  | | |