filemerge: add union-other-first as internal merge tool

filemerge: add union-other-first as internal merge tool

See inline documentation for details.

--- a/mercurial/filemerge.py	Fri Jan 13 00:07:07 2023 +0100
+++ b/mercurial/filemerge.py	Thu Jan 26 00:23:07 2023 +0100
@@ -478,8 +478,9 @@
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will fail if there are any conflicts and leave markers in
-    the partially merged file. Markers will have two sections, one for each side
-    of merge, unless mode equals 'union' which suppresses the markers."""
+    the partially merged file. Markers will have two sections, one for each
+    side of merge, unless mode equals 'union' or 'union-other-first' which
+    suppresses the markers."""
     ui = repo.ui
 
     try:
@@ -510,12 +511,28 @@
 def _iunion(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal non-interactive simple merge algorithm for merging
-    files. It will use both left and right sides for conflict regions.
+    files. It will use both local and other sides for conflict regions by
+    adding local on top of other.
     No markers are inserted."""
     return _merge(repo, local, other, base, b'union')
 
 
 @internaltool(
+    b'union-other-first',
+    fullmerge,
+    _(
+        b"warning: conflicts while merging %s! "
+        b"(edit, then use 'hg resolve --mark')\n"
+    ),
+    precheck=_mergecheck,
+)
+def _iunion_other_first(repo, mynode, local, other, base, toolconf, backup):
+    """
+    Like :union, but add other on top of local."""
+    return _merge(repo, local, other, base, b'union-other-first')
+
+
+@internaltool(
     b'merge',
     fullmerge,
     _(

--- a/mercurial/simplemerge.py	Fri Jan 13 00:07:07 2023 +0100
+++ b/mercurial/simplemerge.py	Thu Jan 26 00:23:07 2023 +0100
@@ -512,6 +512,8 @@
     conflicts = False
     if mode == b'union':
         lines = _resolve(m3, (1, 2))
+    elif mode == b'union-other-first':
+        lines = _resolve(m3, (2, 1))
     elif mode == b'local':
         lines = _resolve(m3, (1,))
     elif mode == b'other':

--- a/relnotes/next	Fri Jan 13 00:07:07 2023 +0100
+++ b/relnotes/next	Thu Jan 26 00:23:07 2023 +0100
@@ -2,6 +2,9 @@
 
 == New Features ==
 
+ * There is a new internal merge tool called `internal:union-other-first`.
+   It works like `internal:union` but add other side on top of local.
+
 == Default Format Change ==
 
 These changes affect newly created repositories (or new clones) done with

--- a/tests/test-help.t	Fri Jan 13 00:07:07 2023 +0100
+++ b/tests/test-help.t	Thu Jan 26 00:23:07 2023 +0100
@@ -2172,8 +2172,11 @@
   
       ":union"
         Uses the internal non-interactive simple merge algorithm for merging
-        files. It will use both left and right sides for conflict regions. No
-        markers are inserted.
+        files. It will use both local and other sides for conflict regions by
+        adding local on top of other. No markers are inserted.
+  
+      ":union-other-first"
+        Like :union, but add other on top of local.
   
       Internal tools are always available and do not require a GUI but will by
       default not handle symlinks or binary files. See next section for detail

--- a/tests/test-merge-internal-tools-pattern.t	Fri Jan 13 00:07:07 2023 +0100
+++ b/tests/test-merge-internal-tools-pattern.t	Thu Jan 26 00:23:07 2023 +0100
@@ -140,3 +140,23 @@
   third line
   line 4b
   line 4a
+
+Merge using internal:union-other-first tool:
+
+  $ hg update -C 4
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo "[merge-patterns]" > .hg/hgrc
+  $ echo "* = internal:union-other-first" >> .hg/hgrc
+
+  $ hg merge 3
+  merging f
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ cat f
+  line 1
+  line 2
+  third line
+  line 4a
+  line 4b