Jeff King authored May 09, 2019 and Junio C Hamano committed May 13, 2019

The "prefix" variable passed by git.c into the builtin cmd_read_tree()
and cmd_write_tree() functions is named "unused_prefix". But we do in
fact pass it to parse_options(), which may use the prefix to adjust any
filename options. Let's get rid of this confusing name.

However, we can't just call it "prefix". The reason these variables were
renamed in the first place is that they shadowed local variables named
"prefix", because these commands both take a "--prefix" option.

So let's rename the parameters, but try to reduce further confusion:

  1. In both cases we'll call them "cmd_prefix" to mark that they're
     part of the cmd_* interface.

  2. In cmd_write_tree(), we'll rename the local prefix variable to
     "tree_prefix" to make it more clear that we're talking about the
     prefix to be used for the tree we're writing.

  3. In cmd_read_tree(), the "prefix" local has since migrated into
     "struct unpack_trees_options". We'll leave that alone, as the
     context within the struct makes its meaning clear (we actually
     _could_ just call the parameter "prefix" now, but that invites
     confusion in the other direction).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>