sdk: introduce the cd subcommand

Typically contributors want to work on specific projects, and to that
end, we now provide a quick way to change the directory to the worktree
of a given project. Example:

	sdk cd git

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

git-extra/git-sdk.sh

@@ -29,9 +29,12 @@ sdk () {
 		create-desktop-icon: install a desktop icon that starts the Git for
 		    Windows SDK Bash.
 
-		init <project>: initialize and/or update a worktree. Known projects
+		cd <project>: initialize/update a worktree and cd into it. Known projects
 		    are: build-extra, git, MINGW-packages, MSYS2-packages.
 
+		init <project>: initialize and/or update a worktree. Known projects
+		    are the same as for the 'cd' command.
+
 		build <project>: builds one of the following: git, git-and-installer.
 		EOF
 		;;
@@ -85,6 +88,11 @@ sdk () {
 			;;
 		esac
 		;;
+	cd)
+		sdk init "$2" &&
+		cd "$src_dir" ||
+		sdk die "Could not change directory to '$2'"
+		;;
 	init)
 		sdk init-lazy "$2" &&
 		git -C "$src_dir" pull origin master