Avoid error if gitk not found on windows-nt
On windows-nt, if gitk is not in the path, then the 'which' produces nothing to stdout (although it does produce an error message to stderr). The magit-git-string call then returns nil, which is not a valid input to file-executable-p, causing an error when trying to autoload anything defined in magit-extras.el. Include 'exe' as a first conjunct to the 'and', short-circuiting the file-executable-p test in this case.
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