strip: clarify comment around bundle create

We may create two bundles, one for the changeset actually stripped, and one
for the changeset affected by the strip we want to keep. (Changesets with a
higher rev number than the striped ones).

For soft stripping, we still need to the first one (used for "restoring" the
changesets), but not the second one.

We start with clarifying which bits of the code is used for what.

mercurial/repair.py

@@ -168,7 +168,7 b' def strip(ui, repo, nodelist, backup=Tru'
         if rev in tostrip:
             updatebm.append(m)
 
-    # create a changegroup for all the branches we need to keep
+    # backup the changeset we are about to strip
     backupfile = None
     node = nodelist[-1]
     if backup:
@@ -177,6 +177,7 b' def strip(ui, repo, nodelist, backup=Tru'
                        vfs.join(backupfile))
         repo.ui.log("backupbundle", "saved backup bundle to %s\n",
                     vfs.join(backupfile))
+    # create a changegroup for all the branches we need to keep
     tmpbundlefile = None
     if saveheads:
         # do not compress temporary bundle if we remove it from disk later