Return a string for `vc-mode-line' to put in the mode line for FILE.
Format:
"BACKEND-REV" if the file is up-to-date
"BACKEND:REV" if the file is edited (or locked by the calling user)
"BACKEND:LOCKER:REV" if the file is locked by somebody else
"BACKEND@REV" if the file was locally added
"BACKEND!REV" if the file contains conflicts or was removed
"BACKEND?REV" if the file is under VC, but is missing
This function assumes that the file is registered.