Add a global option --retry-lock

Fixes restic#719

If the option is passed, restic will wait the specified duration of time
and retry locking the repo every 10 seconds (or more often if the total
timeout is relatively small).

- Play nice with json output
- Reduce wait time in lock tests
- Rework timeout last attempt
- Reduce test wait time to 0.1s
- Use exponential back off for the retry lock
- Don't pass gopts to lockRepo functions
- Use global variable for retry sleep setup
- Exit retry lock on cancel
- Better wording for flag help
- Reorder debug statement
- Refactor tests
- Lower max sleep time to 1m
- Test that we cancel/timeout in time
- Use non blocking sleep function
- Refactor into minDuration func

Co-authored-by: Julian Brost <julian@0x4a42.net>

cmd/restic/global.go

@@ -59,6 +59,7 @@ type GlobalOptions struct {
 	Quiet           bool
 	Verbose         int
 	NoLock          bool
+	RetryLock       time.Duration
 	JSON            bool
 	CacheDir        string
 	NoCache         bool
@@ -115,6 +116,7 @@ func init() {
 	// use empty paremeter name as `-v, --verbose n` instead of the correct `--verbose=n` is confusing
 	f.CountVarP(&globalOptions.Verbose, "verbose", "v", "be verbose (specify multiple times or a level using --verbose=n``, max level/times is 2)")
 	f.BoolVar(&globalOptions.NoLock, "no-lock", false, "do not lock the repository, this allows some operations on read-only repositories")
+	f.DurationVar(&globalOptions.RetryLock, "retry-lock", 0, "retry to lock the repository if it is already locked, takes a value like 5m or 2h (default: no retries)")
 	f.BoolVarP(&globalOptions.JSON, "json", "", false, "set output mode to JSON for commands that support it")
 	f.StringVar(&globalOptions.CacheDir, "cache-dir", "", "set the cache `directory`. (default: use system default cache directory)")
 	f.BoolVar(&globalOptions.NoCache, "no-cache", false, "do not use a local cache")