Merge branch 'mg/prompt-describe-tags'

The command line prompt (in contrib/) learned a new 'tag' style
that can be specified with GIT_PS1_DESCRIBE_STYLE, to describe a
detached HEAD with "git describe --tags".

* mg/prompt-describe-tags:
  git-prompt: add a describe style for any tags

Author: gitster

contrib/completion/git-prompt.sh

@@ -82,6 +82,7 @@
 #     contains      relative to newer annotated tag (v1.6.3.2~35)
 #     branch        relative to newer tag or branch (master~4)
 #     describe      relative to older annotated tag (v1.6.3.1-13-gdd42c2f)
+#     tag           relative to any older tag (v1.6.3.1-13-gdd42c2f)
 #     default       exactly matching tag
 #
 # If you would like a colored hint about the current dirty state, set
@@ -443,6 +444,8 @@ __git_ps1 ()
 					git describe --contains HEAD ;;
 				(branch)
 					git describe --contains --all HEAD ;;
+				(tag)
+					git describe --tags HEAD ;;
 				(describe)
 					git describe HEAD ;;
 				(* | default)