Rather than use the filename provided by INPUT, which could come from user
input, and so could be something that looks like a dashed parameter,
use a .git/object/<sha> filename.
This avoids user input passing through INPUT and back out, with the file
path then passed to a command, which could do something unexpected with
a dashed parameter, or other special parameter.
Added a note in the design about being careful of passing user input to
commands. They still have to be careful of that in general, just not in
this case.
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