largefiles: don't print files as both large and normal in addremove dryruns

largefiles: return the list of added files from addlargefiles() This will be used to exclude largefile candidates from the normal file matcher, which will allow add and addremove dryruns to not print a file as both a normal and a large file.

largefiles: align the output messages for an added file with core methods Core addremove prints the file relative to cwd only if patterns are provided to the command. Core add always prints relative to cwd. Also, both methods print the subrepo prefix when needed. The 'already a largefile' doesn't have an analog in core, but follows the same rules for consistency.