wt-status: use separate variable for result of shorten_unambiguous_ref

wt-status: use separate variable for result of shorten_unambiguous_ref

Store the pointer to the string allocated by shorten_unambiguous_ref in
a dedicated variable, short_base, and keep base unchanged.  A non-const
variable is more appropriate for such an object.  It avoids having to
cast const away on free and stops redefining the meaning of base, making
the code slightly clearer.

Signed-off-by: Rene Scharfe <redacted>
Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>