Capistrano::FileNotFound: Rails assets manifest file not found when using --print-config-variables option

### Steps to reproduce
Do a cold deploy (where the `/current` path does not exist).
Deploy with `cap --print-config-variables deploy`

### Expected behavior
The assets_manifest file should be copied from the current release_path `/releases/20180718040802/public/assets/.sprockets-manifest-abcd` to `/releases/20180718040802/assets_manifest_backup/.sprockets-manifest-abcd`.

See in the error trace below: `ls: cannot access '/home/deploy-user/test-app/current/public/assets/.sprockets-manifest*': No such file or directory`

Since the `/current` path does not yet exist, the backup fails. Maybe(?) the `/current` should not be getting used at all for the backup and instead it should be the release_path that should be used at all times?

### Actual behavior
An error is thrown:

```
WARN Rails assets manifest file not found.
cap aborted!
SSHKit::Runner::ExecuteError: Exception while executing as deploy-user@127.0.0.1: Rails assets manifest file not found.

Caused by:
Capistrano::FileNotFound: Rails assets manifest file not found.

Tasks: TOP => deploy:assets:backup_manifest
(See full trace by running task with --trace)
The deploy has failed with an error: Exception while executing as deploy-user@127.0.0.1: Rails assets manifest file not found.

** DEPLOY FAILED
** Refer to log/capistrano.log for details. Here are the last 20 lines:

DEBUG [35002d68] Finished in 0.101 seconds with exit status 0 (successful).
INFO [16952ff6] Running /usr/bin/env mkdir -p /home/deploy-user/test-app/releases/20180718040802/assets_manifest_backup as deploy-user@127.0.0.1
DEBUG [16952ff6] Command: cd /home/deploy-user/test-app/releases/20180718040802 && /usr/bin/env mkdir -p /home/deploy-user/test-app/releases/20180718040802/assets_manifest_backup
INFO [eeef99ea] Finished in 0.103 seconds with exit status 0 (successful).
DEBUG [c149607f] Running /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/.sprockets-manifest* as deploy-user@172.16.1.153
DEBUG [c149607f] Command: cd /home/deploy-user/test-app/releases/20180718040802 && /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/.sprockets-manifest*
INFO [502f9b11] Finished in 0.101 seconds with exit status 0 (successful).
DEBUG [a569bd45] Running /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/.sprockets-manifest* as deploy-user@127.0.0.1
DEBUG [a569bd45] Command: cd /home/deploy-user/test-app/releases/20180718040802 && /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/.sprockets-manifest*
DEBUG [2f0f2f0c] ls: cannot access '/home/deploy-user/test-app/current/public/assets/.sprockets-manifest*': No such file or directory
DEBUG [2f0f2f0c] Finished in 0.105 seconds with exit status 2 (failed).
DEBUG [e28e7923] Running /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/manifest*.* as deploy-user@127.0.0.1
DEBUG [e28e7923] Command: cd /home/deploy-user/test-app/releases/20180718040802 && /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/manifest*.*
DEBUG [924d8181] ls: cannot access '/home/deploy-user/test-app/current/public/assets/.sprockets-manifest*': No such file or directory
DEBUG [924d8181] Finished in 0.112 seconds with exit status 2 (failed).
DEBUG [8b58a53b] Running /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/manifest*.* as deploy-user@172.16.1.153
DEBUG [8b58a53b] Command: cd /home/deploy-user/test-app/releases/20180718040802 && /usr/bin/env ls /home/deploy-user/test-app/current/public/assets/manifest*.*
DEBUG [d9b7d0fe] ls: cannot access '/home/deploy-user/test-app/current/public/assets/manifest*.*': No such file or directory
DEBUG [d9b7d0fe] Finished in 0.105 seconds with exit status 2 (failed).
WARN Rails assets manifest file not found.
```

## Possible Fix?

```diff
diff --git i/lib/capistrano/tasks/assets.rake w/lib/capistrano/tasks/assets.rake
index f01a62f..de30734 100644
--- i/lib/capistrano/tasks/assets.rake
+++ w/lib/capistrano/tasks/assets.rake
@@ -101,8 +101,14 @@ namespace :deploy do
end
end

+ def assets_manifests_path
+ fetch(:assets_manifests).map do |pattern|
+ release_path.join("public", fetch(:assets_prefix), pattern)
+ end
+ end
+
def detect_manifest_path
- fetch(:assets_manifests).each do |candidate|
+ assets_manifests_path.each do |candidate|
return capture(:ls, candidate).strip.gsub(/(\r|\n)/, ' ') if test(:ls, candidate)
end
msg = 'Rails assets manifest file not found.'
@@ -130,10 +136,6 @@ namespace :load do
task :defaults do
set :assets_roles, fetch(:assets_roles, [:web])
set :assets_prefix, fetch(:assets_prefix, 'assets')
- set :assets_manifests, -> {
- %w[.sprockets-manifest* manifest*.*].map do |pattern|
- release_path.join("public", fetch(:assets_prefix), pattern)
- end
- }
+ set :assets_manifests, %w[.sprockets-manifest* manifest*.*]
end
end
```

Start in lib/capistrano/tasks/assets.rake, especially deploy:assets:backup_manifest and detect_manifest_path. Reproduce the cold deploy with cap --print-config-variables deploy, then compare the manifest candidates with the issue's proposed release_path-based change. Done means the manifest is found and backed up from the current release when no current path exists.

Written by the indexing model from the issue text.