swh-identify: follow symlink by default for paths given as args

For path explicitly given as command line arguments (e.g., foo in swh-identify foo), swh-identify should follow symlinks, rather than returning the checksum of the symlink itself. This would be consistent with the default behavior of most *nix tools. (Note however that we should still //not// follow symlinks for links recursively found below directories given as arguments.)

An additional option should be supported to explicitly require to not follow symlinks for paths given as arguments, in case someone really wants to checksum a symlink as such.

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Migrated from T1135 (view on Phabricator)