On closer inspection it isn't necessary to expand the gitk and perl
executable paths, since a) gitk is a GUI application that doesn't get
called often enough (only once at a time) to warrant being optimized,
and b) magit uses it to set an envvar for git (so git itself handles
locating the executable internally -- much faster than Emacs can (esp
over TRAMP), so it only benefits us to expand magit-git-executable.
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