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

docs/content/docs.md

@@ -1153,6 +1153,14 @@ Note also that `--track-renames` is incompatible with
 `--delete-before` and will select `--delete-after` instead of
 `--delete-during`.
 
+### --track-renames-strategy (hash,modtime) ###
+
+This option changes the matching criteria for `--track-renames` to match
+by any combination of modtime, hash, size. Matchig by size is always enabled
+no matter what option is selected here. This also means
+that it enables `--track-renames` support for encrypted destinations.
+If nothing is specified, the default option is matching by hashes.
+
 ### --delete-(before,during,after) ###
 
 This option allows you to specify when files on your destination are

docs/content/flags.md

@@ -128,6 +128,7 @@ These flags are available for every command.
       --tpslimit float                       Limit HTTP transactions per second to this.
       --tpslimit-burst int                   Max burst of transactions for --tpslimit. (default 1)
       --track-renames                        When synchronizing, track file renames and do a server side move if possible
+      --track-renames-strategy                  When tracking renames, use multiple strategies of hash, modtime
       --transfers int                        Number of file transfers to run in parallel. (default 4)
   -u, --update                               Skip files that are newer on the destination.
       --use-cookies                          Enable session cookiejar.