23.7.2 Version Control and the Mode Line
When you visit a file that is under version control, Emacs indicates
this on the mode line. For example, ‘RCS-1.3’ says that RCS is
used for that file, and the current version is 1.3.
The character between the back-end name and the version number
indicates the version control status of the file. ‘-’ means that
the work file is not locked (if locking is in use), or not modified (if
locking is not in use). ‘:’ indicates that the file is locked, or
that it is modified. If the file is locked by some other user (for
instance, ‘jim’), that is displayed as ‘RCS:jim:1.3’.
When Auto Revert mode (see Reverting) reverts a buffer that is
under version control, it updates the version control information in
the mode line. However, Auto Revert mode may not properly update this
information if the version control status changes without changes to
the work file, from outside the current Emacs session. If you set
t, Auto Revert mode updates
the version control status information every
auto-revert-interval seconds, even if the work file itself is
unchanged. The resulting CPU usage depends on the version control
system, but is usually not excessive.