rewrite: simplify the `retained_extras` extra logic

First, we move the definition of value outside of the rebase extensions, as this
apply to all rebase-like operation and some live in other place (like evolve).

Second we make it a simple set, so that it is easy for an extension to add a new
value in it.

Third, we move the associated logic in core too. That make it easily available
to other extensions.

Fourth we simplify it usage, as the verbose version of the filtering is just a
handful on line long, we are just going to test all the value for updates, so
the Projection overlay is not bringing much here.

Note that, we make it a module level set, is a key is worth preserving it is
probably worth preserving in all cases. This was already the behavior prior to
this change.

hgext/rebase.py

@@ -24,7 +24,6 b' from mercurial.node import ('
     wdirrev,
 )
 from mercurial.pycompat import open
-from mercurial.thirdparty.jaraco.collections import Projection
 from mercurial import (
     bookmarks,
     cmdutil,
@@ -86,19 +85,6 b' def _nothingtorebase():'
     return 1
 
 
-def retained_extras():
-    """
-    Yield the names of the extras to be retained.
-    """
-    # graft
-    yield b'source'
-    yield b'intermediate-source'
-
-
-def _save_extras(ctx, extra):
-    extra.update(Projection(retained_extras(), ctx.extra()))
-
-
 def _savebranch(ctx, extra):
     extra[b'branch'] = ctx.branch()
 
@@ -199,7 +185,7 b' class rebaseruntime:'
         self.date = opts.get('date', None)
 
         e = opts.get('extrafn')  # internal, used by e.g. hgsubversion
-        self.extrafns = [_save_extras]
+        self.extrafns = [rewriteutil.preserve_extras_on_rebase]
         if e:
             self.extrafns = [e]
 

mercurial/rewriteutil.py

@@ -27,6 +27,21 b' from . import ('
 
 NODE_RE = re.compile(br'\b[0-9a-f]{6,64}\b')
 
+# set of extra entry that should survive a rebase-like operation, extensible by extensions
+retained_extras_on_rebase = {
+    b'source',
+    b'intermediate-source',
+}
+
+
+def preserve_extras_on_rebase(old_ctx, new_extra):
+    """preserve the relevant `extra` entry from old_ctx on rebase-like operation"""
+    new_extra.update(
+        (key, value)
+        for key, value in old_ctx.extra().items()
+        if key in retained_extras_on_rebase
+    )
+
 
 def _formatrevs(repo, revs, maxrevs=4):
     """returns a string summarizing revisions in a decent size

setup.py

@@ -1303,7 +1303,6 b' packages = ['
     'mercurial.templates',
     'mercurial.thirdparty',
     'mercurial.thirdparty.attr',
-    'mercurial.thirdparty.jaraco',
     'mercurial.thirdparty.zope',
     'mercurial.thirdparty.zope.interface',
     'mercurial.upgrade_utils',