tests: add test showing tags cache drops filtered heads (issue4550)

The tags cache can lose .hgtags filenode entries for filtered heads. Add
a test demonstrating this (bad) behavior.

tests/test-obsolete-tag-cache.t

@@ -0,0 +1,70 b''
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > rebase=
+  > 
+  > [experimental]
+  > evolution = createmarkers
+  > EOF
+
+Create a repo with some tags
+
+  $ hg init repo
+  $ cd repo
+  $ echo initial > foo
+  $ hg -q commit -A -m initial
+  $ hg tag -m 'test tag' test1
+  $ echo first > first
+  $ hg -q commit -A -m first
+  $ hg tag -m 'test2 tag' test2
+  $ hg -q up -r 0
+  $ echo newhead > newhead
+  $ hg commit -A -m newhead
+  adding newhead
+  created new head
+
+Trigger tags cache population by doing something that accesses tags info
+
+  $ hg log -G -T '{rev}:{node|short} {tags} {desc}\n'
+  @  4:042eb6bfcc49 tip newhead
+  |
+  | o  3:c3cb30f2d2cd  test2 tag
+  | |
+  | o  2:d75775ffbc6b test2 first
+  | |
+  | o  1:5f97d42da03f  test tag
+  |/
+  o  0:55482a6fb4b1 test1 initial
+  
+
+  $ cat .hg/cache/tags
+  4 042eb6bfcc4909bad84a1cbf6eb1ddf0ab587d41
+  3 c3cb30f2d2cd0aae008cc91a07876e3c5131fd22 b3bce87817fe7ac9dca2834366c1d7534c095cf1
+  
+  55482a6fb4b1881fa8f746fd52cf6f096bb21c89 test1
+  d75775ffbc6bca1794d300f5571272879bd280da test2
+
+Create some hidden changesets via a rebase and trigger tags cache
+repopulation
+
+  $ hg -q rebase -s 1 -d 4
+  $ hg log -G -T '{rev}:{node|short} {tags} {desc}\n'
+  o  7:eb610439e10e tip test2 tag
+  |
+  o  6:7b4af00c3c83  first
+  |
+  o  5:43ac2a539b3c  test tag
+  |
+  @  4:042eb6bfcc49  newhead
+  |
+  o  0:55482a6fb4b1 test1 initial
+  
+
+.hgtags filenodes for hidden heads should be visible (issue4550)
+(currently broken)
+
+  $ cat .hg/cache/tags
+  7 eb610439e10e0c6b296f97b59624c2e24fc59e30 b3bce87817fe7ac9dca2834366c1d7534c095cf1
+  
+  55482a6fb4b1881fa8f746fd52cf6f096bb21c89 test1
+  d75775ffbc6bca1794d300f5571272879bd280da test2
+