fs: Add --track-renames-strategy for configurable matching criteria for --track-renames

This commit adds the `--track-renames-strategy` flag which allows the
user to choose the strategy for tracking renames when using the
`--track-renames` flag.

This can be "hash" or "modtime" or both currently.

This, when used with `--track-renames-strategy modtime` enables
support for tracking renames in encrypted remotes.

Fixes #3696
Fixes #2721

fs/config.go

@@ -59,7 +59,8 @@ type ConfigInfo struct {
 	InsecureSkipVerify     bool // Skip server certificate verification
 	DeleteMode             DeleteMode
 	MaxDelete              int64
-	TrackRenames           bool // Track file renames.
+	TrackRenames           bool   // Track file renames.
+	TrackRenamesStrategy   string // Comma separated list of stratgies used to track renames
 	LowLevelRetries        int
 	UpdateOlder            bool // Skip files that are newer on the destination
 	NoGzip                 bool // Disable compression
@@ -145,6 +146,8 @@ func NewConfig() *ConfigInfo {
 	c.MultiThreadCutoff = SizeSuffix(250 * 1024 * 1024)
 	c.MultiThreadStreams = 4
 
+	c.TrackRenamesStrategy = "hash"
+
 	return c
 }