On a case-sensitive file system, files can be added with names that differ
only in case (a "case collision"). This would cause an error on case-insensitive
filesystems. A warning or error is now given for such collisions, depending on
the value of ui.portablefilenames ('warn', 'abort', or 'ignore'):
$ touch file File
$ hg add --config ui.portablefilenames=abort File
abort: possible case-folding collision for File
$ hg add File
warning: possible case-folding collision for File
