rebase: support -X to pass through strategy options

git-rebase calls out to merge strategies, but did not support merge
strategy options so far.  Add this, in the same style used in
git-merge.

Sadly we have to do the full quoting/eval dance here, since
merge-recursive supports the --subtree=<path> option which potentially
contains whitespace.

This patch does not cover git rebase -i, which does not call any merge
strategy directly except in --preserve-merges, and even then only for
merges.

[jc: with a trivial fix-up for 'expr']

Signed-off-by: Mike Lundy <mike@fluffypenguin.org>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -44,6 +44,7 @@ To restore the original branch and stop rebasing run \"git rebase --abort\".
 "
 unset newbase
 strategy=recursive
+strategy_opts=
 do_merge=
 dotest="$GIT_DIR"/rebase-merge
 prec=4
@@ -112,7 +113,7 @@ call_merge () {
 	then
 		export GIT_MERGE_VERBOSITY=1
 	fi
-	git-merge-$strategy "$cmt^" -- "$hd" "$cmt"
+	eval 'git-merge-$strategy' $strategy_opts '"$cmt^" -- "$hd" "$cmt"'
 	rv=$?
 	case "$rv" in
 	0)
@@ -293,6 +294,27 @@ do
 	-M|-m|--m|--me|--mer|--merg|--merge)
 		do_merge=t
 		;;
+	-X*|--strategy-option*)
+		case "$#,$1" in
+		1,-X|1,--strategy-option)
+			usage ;;
+		*,-X|*,--strategy-option)
+			newopt="$2"
+			shift ;;
+		*,--strategy-option=*)
+			newopt="$(expr " $1" : ' --strategy-option=\(.*\)')" ;;
+		*,-X*)
+			newopt="$(expr " $1" : ' -X\(.*\)')" ;;
+		1,*)
+			usage ;;
+		esac
+		strategy_opts="$strategy_opts $(git rev-parse --sq-quote "--$newopt")"
+		do_merge=t
+		if test -n "$strategy"
+		then
+			strategy=recursive
+		fi
+		;;
 	-s=*|--s=*|--st=*|--str=*|--stra=*|--strat=*|--strate=*|\
 		--strateg=*|--strategy=*|\
 	-s|--s|--st|--str|--stra|--strat|--strate|--strateg|--strategy)