There were many places the code relied on the string returned from
getenv() to be non-volatile, which is not true, that have been
corrected.

* jk/save-getenv-result:
  builtin_diff(): read $GIT_DIFF_OPTS closer to use
  merge-recursive: copy $GITHEAD strings
  init: make a copy of $GIT_DIR string
  config: make a copy of $GIT_CONFIG string
  commit: copy saved getenv() result
  get_super_prefix(): copy getenv() result