fbcode_builder: getdeps: add fetch subcommand

Summary:
Adds a command that can be used to trigger a fetch for a project

```
$  ./opensource/fbcode_builder/getdeps.py fetch zstd
Cloning https://github.com/facebook/zstd.git...
 ---
+ git clone --depth=100 https://github.com/facebook/zstd.git /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git
Cloning into '/data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git'...
remote: Enumerating objects: 3816, done.
remote: Counting objects: 100% (3816/3816), done.
remote: Compressing objects: 100% (1415/1415), done.
remote: Total 3816 (delta 2556), reused 3312 (delta 2288), pack-reused 0
Receiving objects: 100% (3816/3816), 2.93 MiB | 9.59 MiB/s, done.
Resolving deltas: 100% (2556/2556), done.
Updating /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git -> v1.3.8
 ---
+ git -C /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git fetch origin v1.3.8
From https://github.com/facebook/zstd
 * tag               v1.3.8     -> FETCH_HEAD
 ---
+ git -C /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git checkout FETCH_HEAD
Note: checking out 'FETCH_HEAD'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 470344d Merge pull request #1479 from facebook/visualTest
 ---
+ git -C /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/repos/github.com-facebook-zstd.git submodule update --init
```

Reviewed By: simpkins

Differential Revision: D14691008

fbshipit-source-id: 3afa391360518a08ebd6ff97f5b8b4993f10c4e8

build/fbcode_builder/getdeps.py

@@ -14,6 +14,8 @@ import os
 import subprocess
 import sys
 
+from getdeps.buildopts import setup_build_options
+from getdeps.load import resolve_manifest_path
 from getdeps.manifest import ManifestParser
 from getdeps.platform import HostType
 from getdeps.subcmd import SubCmd, add_subcommands, cmd
@@ -45,6 +47,25 @@ class ShowHostType(SubCmd):
         return 0
 
 
+@cmd("fetch", "fetch the code for a given project")
+class FetchCmd(SubCmd):
+    def setup_parser(self, parser):
+        parser.add_argument(
+            "project",
+            help=(
+                "name of the project or path to a manifest "
+                "file describing the project"
+            ),
+        )
+
+    def run(self, args):
+        opts = setup_build_options(args)
+        manifest_path = resolve_manifest_path(opts, args.project)
+        manifest = ManifestParser(manifest_path)
+        fetcher = manifest.create_fetcher(opts, ctx={})
+        fetcher.update()
+
+
 def build_argparser():
     common_args = argparse.ArgumentParser(add_help=False)
     common_args.add_argument(