Only update timestamp in XLF-files if modified

[neufeind]

For version-control it would be great to only update the timestamp if an XLF-file has actually changed. Maybe we can just skip rewriting the XLF-file at all (not even updating it with the exact same content) to also avoid updating the file-modification-time on the filesystem.

[nicodh]

We have a configuration to avoid the rewrite of the timestamp:

use static date attribute in xliff files

staticDateInXliffFiles: 2015-10-01T09:00:00Z

But that's not as convenient as your proposal...