bash: Match lightweight tags in prompt

The bash prompt would display a commit's object name when having checked
out a lightweight tag.  Provide `--tags` to `git describe` in the completion
script, so it will display lightweight tag names, as it already does for
annotated tags.

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

Author: knittl

contrib/completion/git-completion.bash

@@ -255,7 +255,7 @@ __git_ps1 ()
 				(describe)
 					git describe HEAD ;;
 				(* | default)
-					git describe --exact-match HEAD ;;
+					git describe --tags --exact-match HEAD ;;
 				esac 2>/dev/null)" ||
 
 				b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||