describe: prepend "tags/" when describing tags with embedded name

describe: prepend "tags/" when describing tags with embedded name

The man page of the "git describe" command explains the expected
output when using the --all option, i.e. the full reference path is
shown, including heads/ or tags/ prefix.

When 212945d4a85dfa172ea55ec73b1d830ef2d8582f ("Teach git-describe
to verify annotated tag names before output") made Git favor the
embedded name of annotated tags, it accidentally changed the output
format when the --all flag is given, only printing the tag's name
without the prefix.

Check if --all was specified and re-add the "tags/" prefix for this
special case to fix the regresssion.

Signed-off-by: Daniel Knittl-Frank <knittl89+git@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>