repoview: do not crash when localtags refers to non existing revisions

This fixes a crash that may happen when using mercurial 3.0.x.

The _gethiddenblockers function assumed that the output of tags.readlocaltags()
was a dict mapping tags to of valid nodes. However this was not necessarily the
case. When a repository had obsolete revisions and had local tag pointing to a
non existing revision was found, many mercurial commands would crash.

This revision fixes the problem by removing any tags from the output of
tags.readlocaltags() which point to invalid nodes.

We may want to add a warning when this happens (although it might be
annoying to get that warning for every command, possibly even more than once per
command).

A test for this problem has been added to test-obsolete.t. Without this fix the
test would output:

  $ hg tags
  abort: 00changelog.i@3816541e5485: no node!
  [255]

Instead of:

  $ hg tags
  tiptag                             2:3816541e5485
  tip                                2:3816541e5485
  visible                            0:193e9254ce7e

mercurial/tags.py

@@ -72,6 +72,15 b' def readlocaltags(ui, repo, alltags, tag'
     filetags = _readtags(
         ui, repo, data.splitlines(), "localtags",
         recode=encoding.fromlocal)
+
+    # remove tags pointing to invalid nodes
+    cl = repo.changelog
+    for t in filetags.keys():
+        try:
+            cl.rev(filetags[t][0])
+        except (LookupError, ValueError):
+            del filetags[t]
+
     _updatetags(filetags, "local", alltags, tagtypes)
 
 def _readtags(ui, repo, lines, fn, recode=None):

tests/test-obsolete.t

@@ -907,3 +907,15 b' Test that a local tag blocks a changeset'
      date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     A
   
+Test that removing a local tag does not cause some commands to fail
+
+  $ hg tag -l -r tip tiptag
+  $ hg tags
+  tiptag                             2:3816541e5485
+  tip                                2:3816541e5485
+  visible                            0:193e9254ce7e
+  $ hg --config extensions.strip= strip -r tip --no-backup
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg tags
+  visible                            0:193e9254ce7e
+  tip                                0:193e9254ce7e