strbuf: remove global variable

As a library that only interacts with other primitives, strbuf should
not utilize the comment_line_char global variable within its
functions. Therefore, add an additional parameter for functions that use
comment_line_char and refactor callers to pass it in instead.
strbuf_stripspace() removes the skip_comments boolean and checks if
comment_line_char is a non-NUL character to determine whether to skip
comments or not.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1283,7 +1283,7 @@ static int parse_mail(struct am_state *state, const char *mail)
 
 	strbuf_addstr(&msg, "\n\n");
 	strbuf_addbuf(&msg, &mi.log_message);
-	strbuf_stripspace(&msg, 0);
+	strbuf_stripspace(&msg, '\0');
 
 	assert(!state->author_name);
 	state->author_name = strbuf_detach(&author_name, NULL);