cat-file: remove unused includes

cat-file: remove unused includes

 - "exec_cmd.h" became unnecessary at b931aa5a (Call builtin ls-tree
   in git-cat-file -p, 2006-05-26), when it changed an earlier code
   that delegated tree display to "ls-tree" via the run_command()
   API (hence needing "exec_cmd.h") to call cmd_ls_tree() directly.
   We should have removed the include in the same commit, but we
   forgot to do so.

 - "diff.h" was added at e5fba602 (textconv: support for cat_file,
   2010-06-15), together with "userdiff.h", but "userdiff.h" can be
   included without including "diff.h"; the header was unnecessary
   from the beginning.

 - "tag.h" and "tree.h" were necessary since 8e440259 (Use blob_,
   commit_, tag_, and tree_type throughout., 2006-04-02) to check
   the type of object by comparing typename with tree_type and
   tag_type (pointers to extern strings).

   21666f1a (convert object type handling from a string to a number,
   2007-02-26) made these <type>_type strings unnecessary, and it
   could have switched to include "object.h", which is necessary to
   use typename(), but it forgot to do so.  Because "tag.h" and
   "tree.h" include "object.h", it did not need to explicitly
   include "object.h" in order to start using typename() itself.

   We do not even have to include "object.h" after removing these
   two #includes, because "builtin.h" includes "commit.h" which in
   turn includes "object.h" these days.  This happened at 7b9c0a69
   (git-commit-tree: make it usable from other builtins,
   2008-07-01).

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>