completion: fix shell expansion of items

As reported by Jeroen Meijer[1]; the current code doesn't deal properly
with items (tags, branches, etc.) that have ${} in them because they get
expaned by bash while using compgen.

A simple solution is to quote the items so they get expanded properly
(\$\{\}).

In order to achieve that I took bash-completion's quote() function,
which is rather simple, and renamed it to __git_quote() as per Jeff
King's suggestion.

Solves the original problem for me.

[1] http://article.gmane.org/gmane.comp.version-control.git/201596

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -225,6 +225,13 @@ _get_comp_words_by_ref ()
 fi
 fi
 
+# Quotes the argument for shell reuse
+__git_quote()
+{
+	local quoted=${1//\'/\'\\\'\'}
+	printf "'%s'" "$quoted"
+}
+
 # Generates completion reply with compgen, appending a space to possible
 # completion words, if necessary.
 # It accepts 1 to 4 arguments:
@@ -261,7 +268,7 @@ __gitcomp ()
 __gitcomp_nl ()
 {
 	local IFS=$'\n'
-	COMPREPLY=($(compgen -P "${2-}" -S "${4- }" -W "$1" -- "${3-$cur}"))
+	COMPREPLY=($(compgen -P "${2-}" -S "${4- }" -W "$(__git_quote "$1")" -- "${3-$cur}"))
 }
 
 __git_heads ()