Merge Scripts: Use relative paths when exporting

[Myaamori]

Right now the absolute path to the file is cached upon import and then used when exporting changes. This means that a merge template containing imported lines is not portable between different systems. Additionally, even if the user changes the import definition to point to a different filename, the script will export to the old location.

Instead, the relative path given in the import definition should be used when exporting as well. Blocked by #4 for the case where the user wishes to export using a different filename.