Really make the LF after reset in fast-import optional

Really make the LF after reset in fast-import optional

cmd_from() ends with a call to read_next_command(), which is needed
when using cmd_from() from commands where from is not the last element.

With reset, however, "from" is the last command, after which the flow
returns to the main loop, which calls read_next_command() again.

Because of this, always set unread_command_buf in cmd_reset_branch(),
even if cmd_from() was successful.

Add a test case for this in t9300-fast-import.sh.

Signed-off-by: Adeodato Simó <dato@net.com.org.es>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>