The ExternalData_LINK_CONTENT option tells ExternalData to convert real
data files it finds into content links and to "stage" the original
content in a ".ExternalData_<algo>_<hash>" file. However, after a data
object has been staged it is possible that a user-provided pattern in
the "REGEX:" option will later match the staged object file. We must
not process staged object files even when a user pattern matches them.
Fix the implementation to not match a staged object file as a normal
data file for conversion. Extend the RunCMake.ExternalData test to
cover this case.
Brad King