'intltool-merge -k' fails to handle absolute paths

$ LC_ALL=C intltool-merge -k -u -c po/.intltool-merge-cache po data/gramps.keys.in /tmp/test/data/gramps.keys
Found cached translation database
Merging translations into /tmp/test/data/gramps.keys.
Cannot open .//tmp/test/data/gramps.keys: No such file or directory

This is the command that is run by gramps setup.py in an out-of-source build, when the build-dir is passed as an absolute path.

The cause of the failure is that intltool implicitly prepends language-based subdirectory to the destination path. I have no idea about multiple file output, so I can't tell if it can be fixed there or not. But I believe the non-multiple-output mode shall be able to handle absolute paths similarly to how options other than '-k' handle it.

I believe that the issue can be fixed quite easily, through prepending $lang only if multiple-output mode is requested, instead of using './' as a dirty hack-around.

I'm attaching a patch made on top of 0.50.2, as I don't see a snapshot download link anywhere. Looking at the online viewer, the code should match on HEAD anyway.