diff tests/test-help.t @ 46108:bdc2bf68f19e

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/tests/test-help.t	Thu Dec 10 14:39:22 2020 -0800
+++ b/tests/test-help.t	Tue Dec 08 16:45:13 2020 -0800
@@ -2057,6 +2057,13 @@
         partially merged file. Marker will have three sections, one from each
         side of the merge and one for the base content.
   
+      ":mergediff"
+        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)
+  
       ":other"
         Uses the other 'p2()' version of files as the merged version.