log: map --removed to walkopts.force_changelog_traversal

This is the flag to forcibly enable the slowpath. I'm not sure if the
slowpath parameter should be merged with this flag, so let's keep it as
an immutable flag for now.

I'll add another flag to support "grep --all-files". These two will be the
flags which aren't directly mapped from the command-line options.

mercurial/logcmdutil.py

@@ -693,6 +693,10 b' class walkopts(object):'
     # 0: no follow, 1: follow first, 2: follow both parents
     follow = attr.ib(default=0)  # type: int
 
+    # do not attempt filelog-based traversal, which may be fast but cannot
+    # include revisions where files were removed
+    force_changelog_traversal = attr.ib(default=False)  # type: bool
+
     # limit number of changes displayed; None means unlimited
     limit = attr.ib(default=None)  # type: Optional[int]
 
@@ -715,6 +719,7 b' def parseopts(ui, pats, opts):'
         opts=opts,
         revspec=opts.get(b'rev', []),
         follow=follow,
+        force_changelog_traversal=bool(opts.get(b'removed')),
         limit=getlimit(opts),
     )
 
@@ -736,7 +741,7 b' def _makematcher(repo, revs, wopts):'
     wctx = repo[None]
     match, pats = scmutil.matchandpats(wctx, wopts.pats, wopts.opts)
     slowpath = match.anypats() or (
-        not match.always() and wopts.opts.get(b'removed')
+        not match.always() and wopts.force_changelog_traversal
     )
     if not slowpath:
         if wopts.follow and wopts.revspec:
@@ -923,6 +928,7 b' def getrevs(repo, wopts):'
     revs = _initialrevs(repo, wopts)
     if not revs:
         return smartset.baseset(), None
+    # TODO: might want to merge slowpath with wopts.force_changelog_traversal
     match, pats, slowpath = _makematcher(repo, revs, wopts)
     wopts = attr.evolve(wopts, pats=pats)
 
@@ -931,6 +937,7 b' def getrevs(repo, wopts):'
         if slowpath or match.always():
             revs = dagop.revancestors(repo, revs, followfirst=wopts.follow == 1)
         else:
+            assert not wopts.force_changelog_traversal
             revs, filematcher = _fileancestors(
                 repo, revs, match, followfirst=wopts.follow == 1
             )