strbuf: remove global variable

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>