backups: add more info, keep 1 of each version opt

backup's filename format has changed, and includes the commit too now. a
Backup struct for going to/from the filename has been added, and the
option "keep 1 backup from each version" has been added, leaving the
most recent backup from each version always. All pre-this-commit backups
are considered the same "old" version.

backups_test.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func testBackupParse(f string, a Backup, t *testing.T) {
+	b := Backup{}
+	err := b.FromString(f)
+	if err != nil {
+		t.Fatalf("error: %+v", err)
+	}
+	if !b.Equals(a) {
+		t.Fatalf("not equal: %+v != %+v", b, a)
+	}
+}
+
+func TestBackupParserOld(t *testing.T) {
+	Q1 := BACKUP_PREFIX + "2023-12-21T21-08-00" + BACKUP_SUFFIX
+	A1 := Backup{}
+	A1.Date, _ = time.Parse(BACKUP_DATEFMT, "2023-12-21T21-08-00")
+	testBackupParse(Q1, A1, t)
+}
+func TestBackupParserOldUpload(t *testing.T) {
+	Q2 := BACKUP_UPLOAD_PREFIX + BACKUP_PREFIX + "2023-12-21T21-08-00" + BACKUP_SUFFIX
+	A2 := Backup{
+		Upload: true,
+	}
+	A2.Date, _ = time.Parse(BACKUP_DATEFMT, "2023-12-21T21-08-00")
+	testBackupParse(Q2, A2, t)
+}
+func TestBackupParserUploadDate(t *testing.T) {
+	Q3 := BACKUP_UPLOAD_PREFIX + BACKUP_PREFIX + BACKUP_DATE_PREFIX + "2023-12-21T21-08-00" + BACKUP_SUFFIX
+	A3 := Backup{
+		Upload: true,
+	}
+	A3.Date, _ = time.Parse(BACKUP_DATEFMT, "2023-12-21T21-08-00")
+	testBackupParse(Q3, A3, t)
+}
+func TestBackupParserUploadCommitDate(t *testing.T) {
+	Q4 := BACKUP_UPLOAD_PREFIX + BACKUP_PREFIX + BACKUP_COMMIT_PREFIX + "testcommit" + BACKUP_DATE_PREFIX + "2023-12-21T21-08-00" + BACKUP_SUFFIX
+	A4 := Backup{
+		Commit: "testcommit",
+		Upload: true,
+	}
+	A4.Date, _ = time.Parse(BACKUP_DATEFMT, "2023-12-21T21-08-00")
+	testBackupParse(Q4, A4, t)
+}
+func TestBackupParserDateCommit(t *testing.T) {
+	Q5 := BACKUP_PREFIX + BACKUP_DATE_PREFIX + "2023-12-21T21-08-00" + BACKUP_COMMIT_PREFIX + "testcommit" + BACKUP_SUFFIX
+	A5 := Backup{
+		Commit: "testcommit",
+	}
+	A5.Date, _ = time.Parse(BACKUP_DATEFMT, "2023-12-21T21-08-00")
+	testBackupParse(Q5, A5, t)
+}