absorb: preserve changesets which were already empty

Most commands in Mercurial (commit, rebase, absorb itself) don’t create empty
changesets or drop them if they become empty. If there’s a changeset that’s
empty, it must be a deliberate choice of the user. At least it shouldn’t be
absorb’s responsibility to prune them. The fact that changesets that became
empty during absorb are pruned, is unaffected by this.

This case was found while writing patches which make it possible to configure
absorb and rebase to not drop empty changesets. Even without having such config
set, I think it’s valuable to preserve changesets which were already empty.

hgext/absorb.py

@@ -782,7 +782,9 b' class fixupstate(object):'
                 # nothing changed, nothing commited
                 nextp1 = ctx
                 continue
-            if self._willbecomenoop(memworkingcopy, ctx, nextp1):
+            if ctx.files() and self._willbecomenoop(
+                memworkingcopy, ctx, nextp1
+            ):
                 # changeset is no longer necessary
                 self.replacemap[ctx.node()] = None
                 msg = _(b'became empty and was dropped')

tests/test-absorb.t

@@ -587,3 +587,21 b' This should move us to the non-obsolete '
       b |  0
       2 files changed, 1 insertions(+), 0 deletions(-)
   
+
+  $ cd ..
+  $ hg init repo8
+  $ cd repo8
+  $ echo a1 > a
+  $ hg commit -m a -A a
+  $ hg commit -m empty --config ui.allowemptycommit=True
+  $ echo a2 > a
+  $ hg absorb --apply-changes --verbose | grep became
+  0:ecf99a8d6699: 1 file(s) changed, became 2:7e3ccf8e2fa5
+  1:97f72456ae0d: 1 file(s) changed, became 3:2df488325d6f
+  $ hg log -T '{rev} {desc}\n' -G --stat
+  @  3 empty
+  |
+  o  2 a
+      a |  1 +
+      1 files changed, 1 insertions(+), 0 deletions(-)
+