stripspace: respect repository config

The way "git stripspace" reads the configuration was not quite
kosher, in that the code forgot to probe for a possibly existing
repository (note: stripspace is designed to be usable outside the
repository as well).  It read .git/config only when it was run from
the top-level of the working tree by accident.  A recent change
b9605bc4f2 ("config: only read .git/config from configured repos",
2016-09-12) stopped reading the repository-local configuration file
".git/config" unless the repository discovery process is done, so
that .git/config is never read even when run from the top-level,
exposing the old bug more.

When rebasing interactively with a commentChar defined in the
current repository's config, the help text at the bottom of the edit
script potentially used an incorrect comment character. This was not
only funny-looking, but also resulted in tons of warnings like this
one:

	Warning: the command isn't recognized in the following line
	 - #

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stripspace.c

@@ -44,8 +44,10 @@ int cmd_stripspace(int argc, const char **argv, const char *prefix)
 	if (argc)
 		usage_with_options(stripspace_usage, options);
 
-	if (mode == STRIP_COMMENTS || mode == COMMENT_LINES)
+	if (mode == STRIP_COMMENTS || mode == COMMENT_LINES) {
+		setup_git_directory_gently(NULL);
 		git_config(git_default_config, NULL);
+	}
 
 	if (strbuf_read(&buf, 0, 1024) < 0)
 		die_errno("could not read the input");

t/t0030-stripspace.sh

@@ -432,7 +432,7 @@ test_expect_success '-c with changed comment char' '
 	test_cmp expect actual
 '
 
-test_expect_failure '-c with comment char defined in .git/config' '
+test_expect_success '-c with comment char defined in .git/config' '
 	test_config core.commentchar = &&
 	printf "= foo\n" >expect &&
 	printf "foo" | (