Adjust shell scripts to use $(cmd) instead of `cmd`.

* ep/shell-command-substitution: (41 commits)
  t5000-tar-tree.sh: use the $( ... ) construct for command substitution
  t4204-patch-id.sh: use the $( ... ) construct for command substitution
  t4119-apply-config.sh: use the $( ... ) construct for command substitution
  t4116-apply-reverse.sh: use the $( ... ) construct for command substitution
  t4057-diff-combined-paths.sh: use the $( ... ) construct for command substitution
  t4038-diff-combined.sh: use the $( ... ) construct for command substitution
  t4036-format-patch-signer-mime.sh: use the $( ... ) construct for command substitution
  t4014-format-patch.sh: use the $( ... ) construct for command substitution
  t4013-diff-various.sh: use the $( ... ) construct for command substitution
  t4012-diff-binary.sh: use the $( ... ) construct for command substitution
  t4010-diff-pathspec.sh: use the $( ... ) construct for command substitution
  t4006-diff-mode.sh: use the $( ... ) construct for command substitution
  t3910-mac-os-precompose.sh: use the $( ... ) construct for command substitution
  t3905-stash-include-untracked.sh: use the $( ... ) construct for command substitution
  t1050-large.sh: use the $( ... ) construct for command substitution
  t1020-subdirectory.sh: use the $( ... ) construct for command substitution
  t1004-read-tree-m-u-wf.sh: use the $( ... ) construct for command substitution
  t1003-read-tree-prefix.sh: use the $( ... ) construct for command substitution
  t1002-read-tree-m-u-2way.sh: use the $( ... ) construct for command substitution
  t1001-read-tree-m-2way.sh: use the $( ... ) construct for command substitution
  ...