conflicts: escape conflict markers by making them longer

If a file contains lines which look like conflict markers, then we need
to make the real conflict markers longer so that the materialized
conflicts can be parsed unambiguously.

When parsing the conflict, we require that the conflict markers in the
file are at least as long as the materialized conflict markers, and we
only parse the longest conflict markers in the file.

For instance, if we have a file explaining the differences between
Jujutsu's conflict markers and Git's conflict markers, it could produce
a conflict with long markers like this:

```
<<<<<<<<<<< Conflict 1 of 1
%%%%%%%%%%% Changes from base to side jj-vcs#1
 Jujutsu uses different conflict markers than Git, which just shows the
-sides of a conflict without a diff.
+sides of a conflict without a diff:
+
+<<<<<<<
+left
+|||||||
+base
+=======
+right
+>>>>>>>
+++++++++++ Contents of side jj-vcs#2
Jujutsu uses different conflict markers than Git:

<<<<<<<
%%%%%%%
-base
+left
+++++++
right
>>>>>>>
>>>>>>>>>>> Conflict 1 of 1 ends
```

CHANGELOG.md

@@ -74,6 +74,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * New `merge-tools.<TOOL>.conflict-marker-style` config option to override the
   conflict marker style used for a specific merge tool.
 
+* Conflict markers are now allowed to be longer than 7 characters, allowing
+  conflicts to be materialized and parsed correctly in files which already
+  contain lines which look like conflict markers.
+
 * `jj simplify-parents` now supports configuring the default revset when no
    `--source` or `--revisions` arguments are provided with the
    `revsets.simplify-parents` config.