Save current buffer in visited file if modified.
Variations are described below.
By default, makes the previous version into a backup file
if previously requested or if this is the first save.
Prefixed with one C-u, marks this version
to become a backup when the next save is done.
Prefixed with two C-u's,
unconditionally makes the previous version into a backup file.
Prefixed with three C-u's, marks this version
to become a backup when the next save is done,
and unconditionally makes the previous version into a backup file.
With a numeric argument of 0, never make the previous version
into a backup file.
If a file's name is FOO, the names of its numbered backup versions are
FOO.~i~ for various integers i. A non-numbered backup file is called FOO~.
Numeric backups (rather than FOO~) will be made if value of
`version-control' is not the atom `never' and either there are already
numeric versions of the file being backed up, or `version-control' is
non-nil.
We don't want excessive versions piling up, so there are variables
`kept-old-versions', which tells Emacs how many oldest versions to keep,
and `kept-new-versions', which tells how many newest versions to keep.
Defaults are 2 old versions and 2 new.
`dired-kept-versions' controls dired's clean-directory (.) command.
If `delete-old-versions' is nil, system will query user
before trimming versions. Otherwise it does it silently.
If `vc-make-backup-files' is nil, which is the default,
no backup files are made for files managed by version control.
(This is because the version control system itself records previous versions.)
See the subroutine `basic-save-buffer' for more information.