Allow setting --commit on slugcmplr release (#88)

`slugcmplr` can be invoked on a separate (but equal) commit to the one
that is eventually released. This can lead to pushing commit IDs which
do not match any commit ID on the default branch for a project.

This can occur especially in scenarios where slugs are pre-emptively
built on auto generated merge-commits between a branch and its target.

This change allows for a manual override of the commit associated with a
release.

README.md

@@ -71,6 +71,9 @@ different from the one you built from. This will work as long as the
 applications are in the same Heroku team. (This is becuase the slug must be
 accessible to the application).
 
+You can optionally pass `--commit [COMMIT]` to associate this release with a
+separate commit from the one used to build this slug initially.
+
 ## Authentication
 
 The `slugcmplr` CLI looks for credentials to `api.heroku.com` in your `.netrc`

cmd/slugcmplr/release.go

@@ -18,7 +18,7 @@ type release struct {
 }
 
 func releaseCmd(verbose bool) *cobra.Command {
-	var buildDir, application string
+	var buildDir, application, commit string
 	cmd := &cobra.Command{
 		Use:   "release",
 		Short: "release a slug",
@@ -49,6 +49,10 @@ func releaseCmd(verbose bool) *cobra.Command {
 				r.Application = application
 			}
 
+			if commit != "" {
+				r.Commit = commit
+			}
+
 			log(out, "application: %v", r.Application)
 			log(out, "slug: %v", r.Slug)
 
@@ -101,6 +105,7 @@ func releaseCmd(verbose bool) *cobra.Command {
 	cmd.Flags().StringVar(&buildDir, "build-dir", "", "The build directory")
 	cmd.MarkFlagRequired("build-dir") // nolint:errcheck
 
+	cmd.Flags().StringVar(&commit, "commit", "", "Override the commit this release is associated with")
 	cmd.Flags().StringVar(&application, "app", "", "Override the application to release to")
 
 	return cmd