checkout: merge local modifications while switching branches.

 * Instead of going interactive, introduce a command line switch
   '-m' to allow merging changes when normal two-way merge by
   read-tree prevents branch switching.

 * Leave the unmerged stages intact if automerge fails, but
   reset index entries of cleanly merged paths to that of the
   new branch, so that "git diff" (not "git diff HEAD") would
   show the local modifications.

 * Swap the order of trees in read-tree three-way merge used in
   the fallback, so that `git diff` to show the conflicts become
   more natural.

 * Describe the new option and give more examples in the documentation.

Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 38 insertions, 22 deletions

diff --git a/git-checkout.sh b/git-checkout.sh
index 76e6a41c6c..bd7f007307 100755
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-USAGE='[-f] [-b <new_branch>] [<branch>] [<paths>...]'
+USAGE='[-f] [-b <new_branch>] [-m] [<branch>] [<paths>...]'
 SUBDIRECTORY_OK=Sometimes
 . git-sh-setup
 
@@ -9,6 +9,7 @@ new=
 force=
 branch=
 newbranch=
+merge=
 while [ "$#" != "0" ]; do
     arg="$1"
     shift
@@ -26,6 +27,9 @@ while [ "$#" != "0" ]; do
 	"-f")
 		force=1
 		;;
+	-m)
+		merge=1
+		;;
 	--)
 		break
 		;;
@@ -71,7 +75,7 @@ done
 
 if test "$#" -ge 1
 then
-	if test '' != "$newbranch$force"
+	if test '' != "$newbranch$force$merge"
 	then
 		die "updating paths and switching branches or forcing are incompatible."
 	fi
@@ -121,32 +125,44 @@ then
 	git-checkout-index -q -f -u -a
 else
     git-update-index --refresh >/dev/null
-    git-read-tree -m -u $old $new || (
-	echo >&2 -n "Try automerge [y/N]? "
-	read yesno
-	case "$yesno" in [yY]*) ;; *) exit 1 ;; esac
-
-	# NEEDSWORK: We may want to reset the index from the $new for
-	# these paths after the automerge happens, but it is not done
-	# yet.  Probably we need to leave unmerged ones alone, and
-	# yank the object name & mode from $new for cleanly merged
-	# paths and stuff them in the index.
-
-	names=`git diff-files --name-only`
-	case "$names" in
-	'')	;;
-	*)
-		echo "$names" | git update-index --remove --stdin ;;
+    merge_error=$(git-read-tree -m -u $old $new 2>&1) || (
+	case "$merge" in
+	'')
+		echo >&2 "$merge_error"
+		exit 1 ;;
 	esac
 
+	# Match the index to the working tree, and do a three-way.
+    	git diff-files --name-only | git update-index --remove --stdin &&
 	work=`git write-tree` &&
-	git read-tree -m -u $old $work $new || exit
+	git read-tree --reset $new &&
+	git checkout-index -f -u -q -a &&
+	git read-tree -m -u $old $new $work || exit
+
 	if result=`git write-tree 2>/dev/null`
 	then
-	    echo >&2 "Trivially automerged." ;# can this even happen?
-	    exit 0
+	    echo >&2 "Trivially automerged."
+	else
+	    git merge-index -o git-merge-one-file -a
 	fi
-	git merge-index -o git-merge-one-file -a
+
+	# Do not register the cleanly merged paths in the index yet.
+	# this is not a real merge before committing, but just carrying
+	# the working tree changes along.
+	unmerged=`git ls-files -u`
+	git read-tree --reset $new
+	case "$unmerged" in
+	'')	;;
+	*)
+		(
+			z40=0000000000000000000000000000000000000000
+			echo "$unmerged" |
+			sed -e 's/^[0-7]* [0-9a-f]* /'"0 $z40 /"
+			echo "$unmerged"
+		) | git update-index --index-info
+		;;
+	esac
+	exit 0
     )
 fi