strbuf: use skip_prefix() in strbuf_addftime()

strbuf: use skip_prefix() in strbuf_addftime()

Use the now common skip_prefix() cascade instead of a case statement to
parse the strftime(3) format in strbuf_addftime().  skip_prefix() parses
the "fmt" pointer and advances it appropriately, making additional
pointer arithmetic unnecessary.  The resulting code is more compact and
consistent with most other strbuf_expand_step() loops.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>