context: follow all branches in blockdescendants()

In the initial implementation of blockdescendants (and thus followlines(...,
descend=True) revset), only the first branch encountered in descending
direction was followed.

Update the algorithm so that all children of a revision ('x' in code) are
considered. Accordingly, we need to prevent a child revision to be yielded
multiple times when it gets visited through different path, so we skip 'i'
when this occurs. Finally, since we now consider all parents of a possible
child touching a given line range, we take care of yielding the child if it
has a diff in specified line range with at least one of its parent (same logic
as blockancestors()).

mercurial/context.py

@@ -1217,16 +1217,18 b' def blockdescendants(fctx, fromline, tol'
     seen = {fctx.filerev(): (fctx, (fromline, toline))}
     for i in fl.descendants([fctx.filerev()]):
         c = fctx.filectx(i)
+        inrange = False
         for x in fl.parentrevs(i):
             try:
-                p, linerange2 = seen.pop(x)
+                p, linerange2 = seen[x]
             except KeyError:
                 # nullrev or other branch
                 continue
-            inrange, linerange1 = _changesrange(c, p, linerange2, diffopts)
-            if inrange:
-                yield c, linerange1
+            inrangep, linerange1 = _changesrange(c, p, linerange2, diffopts)
+            inrange = inrange or inrangep
             seen[i] = c, linerange1
+        if inrange:
+            yield c, linerange1
 
 class committablectx(basectx):
     """A committablectx object provides common functionality for a context that

tests/test-annotate.t

@@ -600,7 +600,28 b' we are missing the branch with rename wh'
   $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 5:7, startrev=25, descend=True)'
   26: baz:3+->3-
 
+we follow all branches in descending direction
+  $ hg up 22 --quiet
+  $ sed 's/3/+3/' baz > baz.new
+  $ mv baz.new baz
+  $ hg ci -m 'baz:3->+3'
+  created new head
+  $ hg log -T '{rev}: {desc}\n' -r 'followlines(baz, 3:5, startrev=16, descend=True)' --graph
+  @  29: baz:3->+3
+  :
+  : o  26: baz:3+->3-
+  : :
+  : o  23: baz:3->3+
+  :/
+  o    20: baz:4
+  |\
+  | ~
+  o  19: baz:3
+  |
+  ~
+
 check error cases
+  $ hg up 23 --quiet
   $ hg log -r 'followlines()'
   hg: parse error: followlines takes at least 1 positional arguments
   [255]