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.

While will never be generated by the vim source code.
http://unix.stackexchange.com/questions/326707/vim-what-are-all-the-possible-swapfile-extensions

Added auto-generated tag files.

Anything matched by `*.un~` will also be matched by the last rule, `*~`.

The old pattern was too broad and was making VIM ignore Scheme (*.scm) files.
Now it only matches files that start with . or _.

.*.s[a-w][a-z] matches named swap files.
.s[a-w][a-z] matches unnamed swap files.

Vim creates buffer swapfiles starting at .swp for buffers that are
active but untitled and unsaved.

Changed line matching vim swapfiles to use all names it will create as described in http://vimdoc.sourceforge.net/htmldoc/recover.html#E326

Include .netrwhist, the user's history and potential privacy concern. See: http://stackoverflow.com/questions/9850360/