revset-filelog: handle hidden linkrev for file missing for head (issue4490)

The fix for linkrev pointing to hidden revision was crashing when the file was
missing from head's manifest. We now properly handle this case.

(yes I feel silly)

mercurial/revset.py

@@ -837,8 +837,9 b' def filelog(repo, subset, x):'
                 if lowesthead is None:
                     lowesthead = {}
                     for h in repo.heads():
-                        fnode = repo[h].manifest()[f]
-                        lowesthead[fl.rev(fnode)] = h
+                        fnode = repo[h].manifest().get(f)
+                        if fnode is not None:
+                            lowesthead[fl.rev(fnode)] = h
                 headrev = lowesthead.get(fr)
                 if headrev is None:
                     # content is nowhere unfiltered

tests/test-log.t

@@ -1744,3 +1744,59 b' Even when a head revision is linkrev-sha'
   
 
   $ cd ..
+
+Even when the file revision is missing from some head:
+
+  $ hg init issue4490
+  $ cd issue4490
+  $ echo '[experimental]' >> .hg/hgrc
+  $ echo 'evolution=createmarkers' >> .hg/hgrc
+  $ echo a > a
+  $ hg ci -Am0
+  adding a
+  $ echo b > b
+  $ hg ci -Am1
+  adding b
+  $ echo B > b
+  $ hg ci --amend -m 1
+  $ hg up 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo c > c
+  $ hg ci -Am2
+  adding c
+  created new head
+  $ hg up 'head() and not .'
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg log -G
+  o  changeset:   4:db815d6d32e6
+  |  tag:         tip
+  |  parent:      0:f7b1eb17ad24
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     2
+  |
+  | @  changeset:   3:9bc8ce7f9356
+  |/   parent:      0:f7b1eb17ad24
+  |    user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     1
+  |
+  o  changeset:   0:f7b1eb17ad24
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     0
+  
+  $ hg log -f -G b
+  @  changeset:   3:9bc8ce7f9356
+  |  parent:      0:f7b1eb17ad24
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     1
+  |
+  $ hg log -G b
+  @  changeset:   3:9bc8ce7f9356
+  |  parent:      0:f7b1eb17ad24
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     1
+  |