revset: rename generationsrel() to generationssubrel()

This highlights that this is an implementation of 'x#y[z]' relation subscript
operator as opposed to 'x#y' relation operator.

Differential Revision: https://phab.mercurial-scm.org/D8393

mercurial/revset.py

@@ -281,7 +281,7 b' def _splitrange(a, b):'
     return ancdepths, descdepths
 
 
-def generationsrel(repo, subset, x, rel, z, order):
+def generationssubrel(repo, subset, x, rel, z, order):
     # TODO: rewrite tests, and drop startdepth argument from ancestors() and
     # descendants() predicates
     a, b = getintrange(
@@ -2650,8 +2650,8 b' methods = {'
 }
 
 subscriptrelations = {
-    b"g": generationsrel,
-    b"generations": generationsrel,
+    b"g": generationssubrel,
+    b"generations": generationssubrel,
 }