Add migration subcommand

src/config.rs

@@ -46,6 +46,14 @@ pub(crate) struct Opt {
     )]
     /// The logfile
     pub(crate) logfile: PathBuf,
+    #[structopt(subcommand)]
+    pub(crate) migrate: Option<Migration>,
+}
+
+#[derive(StructOpt, Debug, Clone, Copy)]
+pub(crate) enum Migration {
+    #[structopt(name = "migrate-commit-count")]
+    CacheCommitCount,
 }
 
 pub(crate) fn init() -> Result<()> {