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 are at
least as long as the materialized conflict markers based on the current
tree. This can lead to some unintuitive edge cases which will be solved
in the next commit.

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
```

We should support these options for "git" conflict marker style as well,
since Git actually does support producing longer conflict markers in
some cases through .gitattributes:

https://git-scm.com/docs/gitattributes#_conflict_marker_size

We may also want to support passing the conflict marker length to merge
tools as well in the future, since Git supports a "%L" parameter to pass
the conflict marker length to merge drivers:

https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver

CHANGELOG.md

@@ -19,6 +19,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New features
 
+* 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.
+
 ### Fixed bugs
 
 * The `$NO_COLOR` environment variable must now be non-empty to be respected.