difftool: fix bug when printing usage

"git difftool -h" reports an error:

	fatal: BUG: setup_git_env called without repository

Defer repository setup so that the help option processing happens before
the repository is initialized.

Add tests to ensure that the basic usage works inside and outside of a
repository.
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Acked-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/difftool.c

@@ -647,10 +647,6 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
-	/* NEEDSWORK: once we no longer spawn anything, remove this */
-	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
-	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
-
 	git_config(difftool_config, NULL);
 	symlinks = has_symlinks;
 
@@ -661,6 +657,10 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	if (tool_help)
 		return print_tool_help();
 
+	/* NEEDSWORK: once we no longer spawn anything, remove this */
+	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
+	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
+
 	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
 		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
 	else if (difftool_cmd) {

t/t7800-difftool.sh

@@ -23,6 +23,19 @@ prompt_given ()
 	test "$prompt" = "Launch 'test-tool' [Y/n]? branch"
 }
 
+test_expect_success 'basic usage requires no repo' '
+	lines=$(git difftool -h | grep ^usage: | wc -l) &&
+	test "$lines" -eq 1 &&
+	# create a ceiling directory to prevent Git from finding a repo
+	mkdir -p not/repo &&
+	ceiling="$PWD/not" &&
+	lines=$(cd not/repo &&
+		GIT_CEILING_DIRECTORIES="$ceiling" git difftool -h |
+		grep ^usage: | wc -l) &&
+	test "$lines" -eq 1 &&
+	rmdir -p not/repo
+'
+
 # Create a file on master and change it on branch
 test_expect_success 'setup' '
 	echo master >file &&