bash: simple reimplementation of _get_comp_words_by_ref

Add a minimal implementation of _get_comp_words_by_ref so

	$ git show head:g <tab><tab>

on bash 4 can complete paths within the head commit without requiring
the bash_completion functions to be loaded.  This is a follow-up to
the previous patch (bash: get --pretty=m<tab> completion to work with
bash v4).

Based on bash-completion 2.x (commit bf763033, 2010-10-26) but tweaked
for simplicity and to allow zsh to parse the code.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: SZEDER Gábor <szeder@ira.uka.de>

contrib/completion/git-completion.bash

@@ -321,26 +321,128 @@ __gitcomp_1 ()
 	done
 }
 
+# The following function is based on code from:
+#
+#   bash_completion - programmable completion functions for bash 3.2+
+#
+#   Copyright © 2006-2008, Ian Macdonald <ian@caliban.org>
+#             © 2009-2010, Bash Completion Maintainers
+#                     <bash-completion-devel@lists.alioth.debian.org>
+#
+#   This program is free software; you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation; either version 2, or (at your option)
+#   any later version.
+#
+#   This program is distributed in the hope that it will be useful,
+#   but WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#   GNU General Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License
+#   along with this program; if not, write to the Free Software Foundation,
+#   Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+#
+#   The latest version of this software can be obtained here:
+#
+#   http://bash-completion.alioth.debian.org/
+#
+#   RELEASE: 2.x
+
+# This function can be used to access a tokenized list of words
+# on the command line:
+#
+#	__git_reassemble_comp_words_by_ref '=:'
+#	if test "${words_[cword_-1]}" = -w
+#	then
+#		...
+#	fi
+#
+# The argument should be a collection of characters from the list of
+# word completion separators (COMP_WORDBREAKS) to treat as ordinary
+# characters.
+#
+# This is roughly equivalent to going back in time and setting
+# COMP_WORDBREAKS to exclude those characters.  The intent is to
+# make option types like --date=<type> and <rev>:<path> easy to
+# recognize by treating each shell word as a single token.
+#
+# It is best not to set COMP_WORDBREAKS directly because the value is
+# shared with other completion scripts.  By the time the completion
+# function gets called, COMP_WORDS has already been populated so local
+# changes to COMP_WORDBREAKS have no effect.
+#
+# Output: words_, cword_, cur_.
+
+__git_reassemble_comp_words_by_ref()
+{
+	local exclude i j first
+	# Which word separators to exclude?
+	exclude="${1//[^$COMP_WORDBREAKS]}"
+	cword_=$COMP_CWORD
+	if [ -z "$exclude" ]; then
+		words_=("${COMP_WORDS[@]}")
+		return
+	fi
+	# List of word completion separators has shrunk;
+	# re-assemble words to complete.
+	for ((i=0, j=0; i < ${#COMP_WORDS[@]}; i++, j++)); do
+		# Append each nonempty word consisting of just
+		# word separator characters to the current word.
+		first=t
+		while
+			[ $i -gt 0 ] &&
+			[ -n "${COMP_WORDS[$i]}" ] &&
+			# word consists of excluded word separators
+			[ "${COMP_WORDS[$i]//[^$exclude]}" = "${COMP_WORDS[$i]}" ]
+		do
+			# Attach to the previous token,
+			# unless the previous token is the command name.
+			if [ $j -ge 2 ] && [ -n "$first" ]; then
+				((j--))
+			fi
+			first=
+			words_[$j]=${words_[j]}${COMP_WORDS[i]}
+			if [ $i = $COMP_CWORD ]; then
+				cword_=$j
+			fi
+			if (($i < ${#COMP_WORDS[@]} - 1)); then
+				((i++))
+			else
+				# Done.
+				return
+			fi
+		done
+		words_[$j]=${words_[j]}${COMP_WORDS[i]}
+		if [ $i = $COMP_CWORD ]; then
+			cword_=$j
+		fi
+	done
+}
+
 if ! type _get_comp_words_by_ref >/dev/null 2>&1; then
 _get_comp_words_by_ref ()
 {
+	local exclude cur_ words_ cword_
+	if [ "$1" = "-n" ]; then
+		exclude=$2
+		shift 2
+	fi
+	__git_reassemble_comp_words_by_ref "$exclude"
+	cur_=${words_[cword_]}
 	while [ $# -gt 0 ]; do
 		case "$1" in
 		cur)
-			cur=${COMP_WORDS[COMP_CWORD]}
+			cur=$cur_
 			;;
 		prev)
-			prev=${COMP_WORDS[COMP_CWORD-1]}
+			prev=${words_[$cword_-1]}
 			;;
 		words)
-			words=("${COMP_WORDS[@]}")
+			words=("${words_[@]}")
 			;;
 		cword)
-			cword=$COMP_CWORD
-			;;
-		-n)
-			# assume COMP_WORDBREAKS is already set sanely
-			shift
+			cword=$cword_
 			;;
 		esac
 		shift