t: add a function to check executable bit

In line with our other helper functions for paths, let's add a function
to check whether a path is executable, and if not, print a suitable
error message.  Document this function, and note that it must only be
used under the POSIXPERM prerequisite, since it doesn't otherwise work
on Windows.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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