mergemaster: handle symbolic links during update.

/etc/os-release is now a symbolic link to a generated file. Make
mergemaster cope with symbolic links generically. I'm no longer
a big mergemaster user, so this has only been lightly tested
by me, though Kimura-san has ran it through its paces.

Submitted by: Yasushiro KIMURA-san
PR: 242212
MFC After: 2 weeks

(cherry picked from commit 30a56f9ef72705f2f3646ce4330cbc20675a465e)