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#3696Fixes#2721
In f544234 we removed the global flags from each command as it was
making each page very big and causing 1000s of lines of duplication in
the man page.
This change adds a new flags page with all the global flags on and
links each command page to it.
Fixes#3273
