util: move diff_parent from logcmdutil to diffutil

util: move diff_parent from logcmdutil to diffutil

This function will be used outside of the log command
(in templatekw, used by hgweb, for which logcmdutil is not available).

Let's move this function together with the rest of the diff-related
utils instead.

--- a/mercurial/commands.py	Thu Sep 07 08:39:21 2023 +0200
+++ b/mercurial/commands.py	Wed Nov 15 02:39:53 2023 +0100
@@ -29,6 +29,7 @@
     copies,
     debugcommands as debugcommandsmod,
     destutil,
+    diffutil,
     discovery,
     encoding,
     error,
@@ -2655,7 +2656,7 @@
     if change:
         repo = scmutil.unhidehashlikerevs(repo, [change], b'nowarn')
         ctx2 = logcmdutil.revsingle(repo, change, None)
-        ctx1 = logcmdutil.diff_parent(ctx2)
+        ctx1 = diffutil.diff_parent(ctx2)
     elif from_rev or to_rev:
         repo = scmutil.unhidehashlikerevs(
             repo, [from_rev] + [to_rev], b'nowarn'

--- a/mercurial/diffutil.py	Thu Sep 07 08:39:21 2023 +0200
+++ b/mercurial/diffutil.py	Wed Nov 15 02:39:53 2023 +0100
@@ -16,6 +16,7 @@
 )
 
 from .i18n import _
+from .node import nullrev
 
 from . import (
     mdiff,
@@ -155,3 +156,35 @@
         )
 
     return mdiff.diffopts(**pycompat.strkwargs(buildopts))
+
+
+def diff_parent(ctx):
+    """get the context object to use as parent when diffing
+
+
+    If diff.merge is enabled, an overlayworkingctx of the auto-merged parents will be returned.
+    """
+    repo = ctx.repo()
+    if repo.ui.configbool(b"diff", b"merge") and ctx.p2().rev() != nullrev:
+        # avoid circular import
+        from . import (
+            context,
+            merge,
+        )
+
+        wctx = context.overlayworkingctx(repo)
+        wctx.setbase(ctx.p1())
+        with repo.ui.configoverride(
+            {
+                (
+                    b"ui",
+                    b"forcemerge",
+                ): b"internal:merge3-lie-about-conflicts",
+            },
+            b"merge-diff",
+        ):
+            with repo.ui.silent():
+                merge.merge(ctx.p2(), wc=wctx)
+        return wctx
+    else:
+        return ctx.p1()

--- a/mercurial/logcmdutil.py	Thu Sep 07 08:39:21 2023 +0200
+++ b/mercurial/logcmdutil.py	Wed Nov 15 02:39:53 2023 +0100
@@ -11,18 +11,18 @@
 import posixpath
 
 from .i18n import _
-from .node import nullrev, wdirrev
+from .node import wdirrev
 
 from .thirdparty import attr
 
 from . import (
     dagop,
+    diffutil,
     error,
     formatter,
     graphmod,
     match as matchmod,
     mdiff,
-    merge,
     patch,
     pathutil,
     pycompat,
@@ -69,35 +69,6 @@
     return limit
 
 
-def diff_parent(ctx):
-    """get the context object to use as parent when diffing
-
-
-    If diff.merge is enabled, an overlayworkingctx of the auto-merged parents will be returned.
-    """
-    repo = ctx.repo()
-    if repo.ui.configbool(b"diff", b"merge") and ctx.p2().rev() != nullrev:
-        # avoid cycle context -> subrepo -> cmdutil -> logcmdutil
-        from . import context
-
-        wctx = context.overlayworkingctx(repo)
-        wctx.setbase(ctx.p1())
-        with repo.ui.configoverride(
-            {
-                (
-                    b"ui",
-                    b"forcemerge",
-                ): b"internal:merge3-lie-about-conflicts",
-            },
-            b"merge-diff",
-        ):
-            with repo.ui.silent():
-                merge.merge(ctx.p2(), wc=wctx)
-        return wctx
-    else:
-        return ctx.p1()
-
-
 def get_diff_chunks(
     ui,
     repo,
@@ -273,7 +244,7 @@
             ui,
             ctx.repo(),
             diffopts,
-            diff_parent(ctx),
+            diffutil.diff_parent(ctx),
             ctx,
             match=self._makefilematcher(ctx),
             stat=stat,
@@ -286,7 +257,7 @@
             ui,
             ctx.repo(),
             diffopts,
-            diff_parent(ctx),
+            diffutil.diff_parent(ctx),
             ctx,
             match=self._makefilematcher(ctx),
             stat=stat,