git sync: unhide jj git sync

## Summary
* [X] Get branch pre-fetch heads
* [X] Build candidates from prefetch heads
* [X] Fetch from remotes
* [X] Get branch post-fetch heads
* [X] Rebase
  * [X] Build old -> new map
  * [X] transform descendants


## Details
* Add tests
* Unhide the command, now that it's complete.
* Update CHANGELOG

Issue: #1039

CHANGELOG.md

@@ -19,6 +19,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New features
 
+* Implement `jj git sync`.
+
 * Templates now support the `==` and `!=` logical operators for `Boolean`,
   `Integer`, and `String` types.
 

cli/tests/cli-reference@.md.snap

@@ -59,6 +59,7 @@ This document contains the help content for the `jj` command-line program.
 * [`jj git remote remove`↴](#jj-git-remote-remove)
 * [`jj git remote rename`↴](#jj-git-remote-rename)
 * [`jj git remote set-url`↴](#jj-git-remote-set-url)
+* [`jj git sync`↴](#jj-git-sync)
 * [`jj help`↴](#jj-help)
 * [`jj init`↴](#jj-init)
 * [`jj interdiff`↴](#jj-interdiff)
@@ -1049,6 +1050,7 @@ For a comparison with Git, including a table of commands, see https://martinvonz
 * `init` — Create a new Git backed repo
 * `push` — Push to a Git remote
 * `remote` — Manage Git remotes
+* `sync` — Sync the local JJ repo to specified Git remote branch(es)
 
 
 
@@ -1263,6 +1265,28 @@ Set the URL of a Git remote
 
 
 
+## `jj git sync`
+
+Sync the local JJ repo to specified Git remote branch(es).
+
+The sync command will first fetch from the Git remote, then rebase all local changes onto the appropriate updated heads that were fetched.
+
+Changes that are made empty by the rebase are dropped.
+
+**Usage:** `jj git sync [OPTIONS]`
+
+###### **Options:**
+
+* `-b`, `--branch <BRANCH>` — Fetch only some of the branches
+
+   By default, the specified name matches exactly. Use `glob:` prefix to expand `*` as a glob. The other wildcard characters aren't supported.
+
+  Default value: `glob:*`
+* `--remote <REMOTE>` — The remote to fetch from (only named remotes are supported, can be repeated)
+* `--all-remotes` — Fetch from all remotes
+
+
+
 ## `jj help`
 
 Print this message or the help of the given subcommand(s)