bash completion: Add long options for 'git describe'

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 40b3d99..2fb88a8 100755
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -667,6 +667,15 @@
 
 _git_describe ()
 {
+	local cur="${COMP_WORDS[COMP_CWORD]}"
+	case "$cur" in
+	--*)
+		__gitcomp "
+			--all --tags --contains --abbrev= --candidates=
+			--exact-match --debug --long --match --always
+			"
+		return
+	esac
 	__gitcomp "$(__git_refs)"
 }