Fix overzealous cleanliness check in git-merge

Being able to try multiple strategies and automatically picking one
that seems to give less conflicting result may or may not much sense
in practice.  At least that should not force normal use case to
additionally require the working tree to be fully clean.  As Linus
shouted, local changes do not matter unless they interfere with the
merge.

This commit changes git-merge not to require a clean working tree.
Only when we will iterate through more than one merge strategies,
local changes are stashed away before trying the first merge, and
restored before second and later merges are attempted.

The index file must be in sync with HEAD in any case -- otherwise the
merge result would contain changes since HEAD that was done locally
and registered in the index.  This check is already enforced by
three-way read-tree existing merge strategies use, but is done here as
a safeguard as well.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-merge.sh

@@ -18,8 +18,19 @@ all_strategies='recursive octopus resolve stupid'
 default_strategies='resolve octopus'
 use_strategies=
 
-dropheads() {
-	rm -f -- "$GIT_DIR/MERGE_HEAD" || exit 1
+dropsave() {
+	rm -f -- "$GIT_DIR/MERGE_HEAD" \
+		 "$GIT_DIR/MERGE_SAVE" || exit 1
+}
+
+savestate() {
+	git diff -r -z --name-only $head | cpio -0 -o >"$GIR_DIR/MERGE_SAVE"
+}
+
+restorestate() {
+	git reset --hard $head
+	cpio -iuv <"$GIT_DIR/MERGE_SAVE"
+	git-update-index --refresh >/dev/null
 }
 
 summary() {
@@ -93,7 +104,7 @@ case "$#,$common" in
 	# If head can reach all the merge then we are up to date.
 	# but first the most common case of merging one remote
 	echo "Already up-to-date. Yeeah!"
-	dropheads
+	dropsave
 	exit 0
 	;;
 1,"$head")
@@ -103,7 +114,7 @@ case "$#,$common" in
 	git-read-tree -u -m $head "$1" || exit 1
 	git-rev-parse --verify "$1^0" > "$GIT_DIR/HEAD"
 	summary "$1"
-	dropheads
+	dropsave
 	exit 0
 	;;
 1,*)
@@ -125,30 +136,51 @@ case "$#,$common" in
 	if test "$up_to_date" = t
 	then
 		echo "Already up-to-date. Yeeah!"
-		dropheads
+		dropsave
 		exit 0
 	fi
 	;;
 esac
 
-# At this point we need a real merge.  Require that the tree matches
-# exactly our head.
+# At this point, we need a real merge.  No matter what strategy
+# we use, it would operate on the index, possibly affecting the
+# working tree, and when resolved cleanly, have the desired tree
+# in the index -- this means that the index must be in sync with
+# the $head commit.
+files=$(git-diff-index --cached --name-only $head) || exit
+if [ "$files" ]; then
+   echo >&2 "Dirty index: cannot merge (dirty: $files)"
+   exit 1
+fi
 
-git-update-index --refresh &&
-test '' = "`git-diff-index --cached --name-only $head`" || {
-	die "Need real merge but the working tree has local changes."
-}
+case "$use_strategies" in
+?*' '?*)
+    # Stash away the local changes so that we can try more than one.
+    savestate
+    single_strategy=no
+    ;;
+*)
+    single_strategy=yes
+    ;;
+esac
 
 result_tree= best_cnt=-1 best_strategy= wt_strategy=
 for strategy in $use_strategies
 do
     test "$wt_strategy" = '' || {
 	echo "Rewinding the tree to pristine..."
-	git reset --hard $head
+	restorestate
     }
-    echo "Trying merge strategy $strategy..."
+    case "$single_strategy" in
+    no)
+	echo "Trying merge strategy $strategy..."
+	;;
+    esac
+
+    # Remember which strategy left the state in the working tree
     wt_strategy=$strategy
-    git-merge-$strategy $common -- $head_arg "$@" || {
+
+    git-merge-$strategy $common -- "$head_arg" "$@" || {
 
 	# The backend exits with 1 when conflicts are left to be resolved,
 	# with 2 when it does not handle the given merge at all.
@@ -186,14 +218,14 @@ then
     echo "Committed merge $result_commit, made by $wt_strategy."
     echo $result_commit >"$GIT_DIR/HEAD"
     summary $result_commit
-    dropheads
+    dropsave
     exit 0
 fi
 
 # Pick the result from the best strategy and have the user fix it up.
 case "$best_strategy" in
 '')
-	git reset --hard $head
+	restorestate
 	die "No merge strategy handled the merge."
 	;;
 "$wt_strategy")
@@ -201,9 +233,9 @@ case "$best_strategy" in
 	;;
 *)
 	echo "Rewinding the tree to pristine..."
-	git reset --hard $head
+	restorestate
 	echo "Using the $best_strategy to prepare resolving by hand."
-	git-merge-$best_strategy $common -- $head_arg "$@"
+	git-merge-$best_strategy $common -- "$head_arg" "$@"
 	;;
 esac
 for remote