A speciality of the rsync tool is that it behaves differently
if the given source_dir ends with a '/' or not. If it ends
with a slash the data structure below will be synced to the
target_dir. If it does not end with a slash the source_dir
and its contents are synced to the target_dir. For example:
source
└── some_data
1. $ rsync -a source target
target
└── source
└── some_data
2. $ rsync -a source/ target
target
└── some_data
The parameter force_trailing_slash in the DataSync::sync_data
method can be used to make sure rsync behaves like shown in
the second case. This Fixes#1786
