mergetools: add new conflict marker format with diffs in I use 3-way conflict markers. Often when I resolve them, I manually compare one the base with one side and apply the differences to the other side. That can be hard when the conflict marker is large. This patch introduces a new type of conflict marker, which I'm hoping will make it easier to resolve conflicts. The new format uses `<<<<<<<` and `>>>>>>>` to open and close the markers, just like our existing 2-way and 3-way conflict markers. Instead of having 2 or 3 snapshots (left+right or left+base+right), it has a sequence of diffs. A diff looks like this: ``` ------- base +++++++ left a -b +c d ``` A diff that adds one side ("diff from nothing") has a `=======` header instead and does not have have `+` prefixed on its lines. A regular 3-way merge can be viewed as adding one side plus a diff between the base and the other side. It thus has two ways of being represented, depending on which side is being diffed: ``` <<<<<<< ======= left contents on left ------- base +++++++ right contents on -left +right >>>>>>> ``` or ``` <<<<<<< ------- base +++++++ left contents on -right +left ======= right contents on right >>>>>>> ``` I've made it so the new merge tool tries to pick a version that has the most common lines (no difference in the example above). I've called the new tool "mergediff" to stick to the convention of starting with "merge" if the tool tries a regular 3-way merge. The idea came from my pet VCS (placeholder name `jj`), which has support for octopus merges and other ways of ending up with merges of more than 3 versions. I wanted to be able to represent such conflicts in the working copy and therefore thought of this format (although I have not yet implemented it in my VCS). I then attended a meeting with Larry McVoy, who said BitKeeper has an option (`bk smerge -g`) for showing a similar format, which reminded me to actually attempt this in Mercurial. Differential Revision: https://phab.mercurial-scm.org/D9551

--- a/mercurial/filemerge.py	Thu Dec 10 14:39:22 2020 -0800
+++ b/mercurial/filemerge.py	Tue Dec 08 16:45:13 2020 -0800
@@ -532,6 +532,33 @@
     return _imerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels)
 
 
+@internaltool(
+    b'mergediff',
+    fullmerge,
+    _(
+        b"warning: conflicts while merging %s! "
+        b"(edit, then use 'hg resolve --mark')\n"
+    ),
+    precheck=_mergecheck,
+)
+def _imerge_diff(
+    repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None
+):
+    """
+    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. The marker will have two sections, one with the
+    content from one side of the merge, and one with a diff from the base
+    content to the content on the other side. (experimental)"""
+    if not labels:
+        labels = _defaultconflictlabels
+    if len(labels) < 3:
+        labels.append(b'base')
+    return _merge(
+        repo, mynode, orig, fcd, fco, fca, toolconf, files, labels, b'mergediff'
+    )
+
+
 def _imergeauto(
     repo,
     mynode,