environment: fix typo: 'setup_git_directory_gently'

environment: fix typo: 'setup_git_directory_gently'

Above the declaration of git_work_tree_cfg, we have:

  /* This is set by setup_git_dir_gently() and/or git_default_config() */
  char *git_work_tree_cfg;

It can be verified that there is no function called
'setup_git_dir_gently' by running grep on the codebase:

  $ grep -R setup_git_dir_gently .
  ./environment.c:/* This is set by setup_git_dir_gently() and/or git_default_config() */

The comment, introduced in e90fdc39b6 (Clean up work-tree handling), is
the only occurrence of the name 'setup_git_dir_gently'.

It probably meant 'setup_git_directory_gently' as that is a name of a
real function in setup.c. Correct it.

Signed-off-by: Abhijeet Sonar <abhijeet.nkt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>