setenv(GIT_DIR) clean-up

This patch converts the setenv() calls in path.c and setup.c.  After
the call, git grep with a pager works again in bare repos.

It leaves the setenv(GIT_DIR_ENVIRONMENT, ...) calls in git.c alone, as
they respond to command line switches that emulate the effect of setting
the environment variable directly.

The remaining site in environment.c is in set_git_dir() and is left
alone, too, of course.  Finally, builtin-init-db.c is left changed
because the repo is still being carefully constructed when the
environment variable is set.

This fixes git shortlog when run inside a git directory, which had been
broken by abe549e1.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -336,7 +336,7 @@ char *enter_repo(char *path, int strict)
 
 	if (access("objects", X_OK) == 0 && access("refs", X_OK) == 0 &&
 	    validate_headref("HEAD") == 0) {
-		setenv(GIT_DIR_ENVIRONMENT, ".", 1);
+		set_git_dir(".");
 		check_repository_format();
 		return path;
 	}

setup.c

@@ -404,9 +404,9 @@ const char *setup_git_directory_gently(int *nongit_ok)
 				inside_work_tree = 0;
 			if (offset != len) {
 				cwd[offset] = '\0';
-				setenv(GIT_DIR_ENVIRONMENT, cwd, 1);
+				set_git_dir(cwd);
 			} else
-				setenv(GIT_DIR_ENVIRONMENT, ".", 1);
+				set_git_dir(".");
 			check_repository_format_gently(nongit_ok);
 			return NULL;
 		}