describe: load refnames before calling describe()

describe: load refnames before calling describe()

Get rid of the static variable that was used to prevent loading all
the refnames multiple times by moving that code out of describe(),
simply making sure it is only run once that way.

Also change the error message that is shown in case no refnames are
found to not include a hash any more, as the error condition is not
specific to any particular revision.

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