context: add a blockdescendants function

This is symmetrical with blockancestors() and yields descendants of a filectx
with changes in the given line range. The noticeable difference is that the
algorithm does not follow renames (probably because filelog.descendants() does
not), so we are missing branches with renames.

mercurial/context.py

@@ -1208,6 +1208,26 b' def blockancestors(fctx, fromline, tolin'
         if inrange:
             yield c, linerange2
 
+def blockdescendants(fctx, fromline, toline):
+    """Yield descendants of `fctx` with respect to the block of lines within
+    `fromline`-`toline` range.
+    """
+    diffopts = patch.diffopts(fctx._repo.ui)
+    fl = fctx.filelog()
+    seen = {fctx.filerev(): (fctx, (fromline, toline))}
+    for i in fl.descendants([fctx.filerev()]):
+        c = fctx.filectx(i)
+        for x in fl.parentrevs(i):
+            try:
+                p, linerange2 = seen.pop(x)
+            except KeyError:
+                # nullrev or other branch
+                continue
+            inrange, linerange1 = _changesrange(c, p, linerange2, diffopts)
+            if inrange:
+                yield c, linerange1
+            seen[i] = c, linerange1
+
 class committablectx(basectx):
     """A committablectx object provides common functionality for a context that
     wants the ability to commit, e.g. workingctx or memctx."""