from vim's documentation on `:mksession` (:help :mksession):
...
10. If a file exists with the same name as the Session file, but ending
in "x.vim" (for eXtra), executes that as well. You can use *x.vim
files to specify additional settings and actions associated with a
given Session, such as creating menu items in the GUI version.
we already have Session.vim ignored. the Sessionx.vim file, like
Session.vim, is a user file. a user would generally want that file kept
private or for themselves, and the public or a team fetching from or
sharing the repository generally have no interest in a file relevant
only to a particular user. so it's a good idea to get git to help us
avoid mistakenly sharing the file.
Rolf Vidar Hoksaas