Support passing a short-sha version for rollback when using git.

Only for emergencies.

lib/kamal/cli/main.rb

@@ -93,7 +93,7 @@ class Kamal::Cli::Main < Kamal::Cli::Base
       mutating do
         invoke_options = deploy_options
 
-        KAMAL.config.version = version
+        KAMAL.config.version = Kamal::Git.used? ? Kamal::Git.resolve_revision(version) : version
         old_version = nil
 
         if container_available?(version)

lib/kamal/git.rb

@@ -13,6 +13,16 @@ module Kamal::Git
     `git rev-parse HEAD`.strip
   end
 
+  # Attempt to convert a short -> long git sha, or return the original
+  def resolve_revision(revision)
+    resolved_rev = `git rev-parse -q --verify #{revision}`.strip
+    if resolved_rev.empty?
+      revision
+    else
+      resolved_rev
+    end
+  end
+
   def uncommitted_changes
     `git status --porcelain`.strip
   end