conflicts.rs: label conflict number and sides next to conflict markers

For example, 

```
<<<<<<< Conflict 1 of 3
+++++++ Contents of side #1
left 3.1
left 3.2
left 3.3
%%%%%%% Changes from base to side #2
-line 3
+right 3.1
>>>>>>>
```

or

```
<<<<<<< Conflict 1 of 1
%%%%%%% Changes from base to side #1
-line 3
+right 3.1
+++++++ Contents of side #2
left 3.1
left 3.2
left 3.3
>>>>>>>
```

Currently, there is no way to disable these, this is TODO for a future
PR. Other TODOs for future PRs: make these labels configurable. After
that, we could support a `diff3/git`-like conflict format as well, in
principle.

Counting conflicts helps with knowing whether you fixed all the
conflicts while you are in the editor.

While labeling "side #1", etc, does not tell you the commit id or
description as requested in #1176, I still think it's an improvement.
Most importantly, I hope this will make `jj`'s conflict format less
scary-looking for new users.

I've used this for a bit, and I like it. Without the labels, I would see
that the two conflicts have a different order of conflict markers, but I
wouldn't be able to remember what that means. For longer diffs, it can
be tricky for me to quickly tell that it's a diff as opposed to one of
the sides. This also creates some hope of being able to navigate a
conflict with more than 2 sides.

Another not-so-secret goal for this is explained in
#3109 (comment). The
idea is a little weird, but I *think* it could be helpful, and I'd like
to experiment with it.

CHANGELOG.md

@@ -26,6 +26,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * A new config option `ui.always-allow-large-revsets` has been added to
   allow large revsets expressions in some commands, without the `all:` prefix.
 
+* Conflict markers now include an explanation of what each part of the conflict
+  represents.
+
 * New command `jj parallelize` that rebases a set of revisions into siblings.
 
 * `jj status` now supports filtering by paths. For example, `jj status .` will

cli/tests/test_cat_command.rs

@@ -89,11 +89,11 @@ fn test_cat() {
     test_env.jj_cmd_ok(&repo_path, &["rebase", "-r", "@", "-d", "@--"]);
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "file1"]);
     insta::assert_snapshot!(stdout, @r###"
-    <<<<<<<
-    %%%%%%%
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base to side #1
     -b
     +a
-    +++++++
+    +++++++ Contents of side #2
     c
     >>>>>>>
     "###);

cli/tests/test_chmod_command.rs

@@ -71,11 +71,11 @@ fn test_chmod_regular_conflict() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "file"]);
     insta::assert_snapshot!(stdout, 
     @r###"
-    <<<<<<<
-    %%%%%%%
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base to side #1
     -base
     +x
-    +++++++
+    +++++++ Contents of side #2
     n
     >>>>>>>
     "###);
@@ -90,11 +90,11 @@ fn test_chmod_regular_conflict() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "file"]);
     insta::assert_snapshot!(stdout, 
     @r###"
-    <<<<<<<
-    %%%%%%%
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base to side #1
     -base
     +x
-    +++++++
+    +++++++ Contents of side #2
     n
     >>>>>>>
     "###);
@@ -107,11 +107,11 @@ fn test_chmod_regular_conflict() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "file"]);
     insta::assert_snapshot!(stdout, 
     @r###"
-    <<<<<<<
-    %%%%%%%
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base to side #1
     -base
     +x
-    +++++++
+    +++++++ Contents of side #2
     n
     >>>>>>>
     "###);
@@ -201,10 +201,10 @@ fn test_chmod_file_dir_deletion_conflicts() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "-r=file_deletion", "file"]);
     insta::assert_snapshot!(stdout,
     @r###"
-    <<<<<<<
-    +++++++
+    <<<<<<< Conflict 1 of 1
+    +++++++ Contents of side #1
     a
-    %%%%%%%
+    %%%%%%% Changes from base to side #2
     -base
     >>>>>>>
     "###);
@@ -234,10 +234,10 @@ fn test_chmod_file_dir_deletion_conflicts() {
     let stdout = test_env.jj_cmd_success(&repo_path, &["cat", "-r=file_deletion", "file"]);
     insta::assert_snapshot!(stdout,
     @r###"
-    <<<<<<<
-    +++++++
+    <<<<<<< Conflict 1 of 1
+    +++++++ Contents of side #1
     a
-    %%%%%%%
+    %%%%%%% Changes from base to side #2
     -base
     >>>>>>>
     "###);

cli/tests/test_diffedit_command.rs

@@ -395,11 +395,11 @@ fn test_diffedit_merge() {
     assert!(!repo_path.join("file1").exists());
     let stdout = test_env.jj_cmd_success(&repo_path, &["print", "file2"]);
     insta::assert_snapshot!(stdout, @r###"
-    <<<<<<<
-    %%%%%%%
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base to side #1
     -a
     +c
-    +++++++
+    +++++++ Contents of side #2
     b
     >>>>>>>
     "###);

cli/tests/test_interdiff_command.rs

@@ -153,11 +153,11 @@ fn test_interdiff_conflicting() {
     --- a/file
     +++ b/file
     @@ -1,7 +1,1 @@
-    -<<<<<<<
-    -%%%%%%%
+    -<<<<<<< Conflict 1 of 1
+    -%%%%%%% Changes from base to side #1
     --foo
     -+abc
-    -+++++++
+    -+++++++ Contents of side #2
     -bar
     ->>>>>>>
     +def

cli/tests/test_obslog_command.rs

@@ -59,10 +59,10 @@ fn test_obslog_with_or_without_diff() {
     @  rlvkpnrz [email protected] 2001-02-03 08:05:10 66b42ad3
     │  my description
     │  Resolved conflict in file1:
-    │     1    1: <<<<<<<resolved
-    │     2     : %%%%%%%
+    │     1    1: <<<<<<< Conflict 1 of 1resolved
+    │     2     : %%%%%%% Changes from base to side #1
     │     3     : -foo
-    │     4     : +++++++
+    │     4     : +++++++ Contents of side #2
     │     5     : foo
     │     6     : bar
     │     7     : >>>>>>>
@@ -111,10 +111,10 @@ fn test_obslog_with_or_without_diff() {
     --- a/file1
     +++ b/file1
     @@ -1,7 +1,1 @@
-    -<<<<<<<
-    -%%%%%%%
+    -<<<<<<< Conflict 1 of 1
+    -%%%%%%% Changes from base to side #1
     --foo
-    -+++++++
+    -+++++++ Contents of side #2
     -foo
     -bar
     ->>>>>>>