config_parse: add comment to clarify logic getting next character

Upon each line, the configuration parser tries to get either the first
non-whitespace character or the first whitespace character, in case
there is no non-whitespace character. The logic handling this looks
rather odd and doesn't immediately convey this meaning, so add a comment
to clarify what happens.

src/config_parse.c

@@ -475,6 +475,11 @@ int git_config_parse(
 		size_t line_len = parser->ctx.line_len;
 		char c;
 
+		/*
+		 * Get either first non-whitespace character or, if that does
+		 * not exist, the first whitespace character. This is required
+		 * to preserve whitespaces when writing back the file.
+		 */
 		if (git_parse_peek(&c, ctx, GIT_PARSE_PEEK_SKIP_WHITESPACE) < 0 &&
 		    git_parse_peek(&c, ctx, 0) < 0)
 			continue;