Return non-nil if there is a master corresponding to FILE.
TEMPLATES is a list of strings or functions. If an element is a
string, it must be a control string as required by `format', with two
string placeholders, such as "%sRCS/%s,v". The directory part of
FILE is substituted for the first placeholder, the basename of FILE
for the second. If a file with the resulting name exists, it is taken
as the master of FILE, and returned.
If an element of TEMPLATES is a function, it is called with the
directory part and the basename of FILE as arguments. It should
return non-nil if it finds a master; that value is then returned by
this function.