merge: loosen overcautious "working file will be lost" check.

The three-way merge complained unconditionally when a path that
does not exist in the index is involved in a merge when it
existed in the working tree.  If we are merging an old version
that had that path tracked, but the path is not tracked anymore,
and if we are merging that old version in, the result will be
that the path is not tracked.  In that case we should not
complain.

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

git-merge-one-file.sh

@@ -23,6 +23,12 @@ case "${1:-.}${2:-.}${3:-.}" in
 "$1.." | "$1.$1" | "$1$1.")
 	if [ "$2" ]; then
 		echo "Removing $4"
+	else
+		# read-tree checked that index matches HEAD already,
+		# so we know we do not have this path tracked.
+		# there may be an unrelated working tree file here,
+		# which we should just leave unmolested.
+		exit 0
 	fi
 	if test -f "$4"; then
 		rm -f -- "$4" &&
@@ -34,8 +40,16 @@ case "${1:-.}${2:-.}${3:-.}" in
 #
 # Added in one.
 #
-".$2." | "..$3" )
+".$2.")
+	# the other side did not add and we added so there is nothing
+	# to be done.
+	;;
+"..$3")
 	echo "Adding $4"
+	test -f "$4" || {
+		echo "ERROR: untracked $4 is overwritten by the merge."
+		exit 1
+	}
 	git-update-index --add --cacheinfo "$6$7" "$2$3" "$4" &&
 		exec git-checkout-index -u -f -- "$4"
 	;;

t/t1004-read-tree-m-u-wf.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+
+test_description='read-tree -m -u checks working tree files'
+
+. ./test-lib.sh
+
+# two-tree test
+
+test_expect_success 'two-way setup' '
+
+	echo >file1 file one &&
+	echo >file2 file two &&
+	git update-index --add file1 file2 &&
+	git commit -m initial &&
+
+	git branch side &&
+	git tag -f branch-point &&
+
+	echo file2 is not tracked on the master anymore &&
+	rm -f file2 &&
+	git update-index --remove file2 &&
+	git commit -a -m "master removes file2"
+'
+
+test_expect_success 'two-way not clobbering' '
+
+	echo >file2 master creates untracked file2 &&
+	if err=`git read-tree -m -u master side 2>&1`
+	then
+		echo should have complained
+		false
+	else
+		echo "happy to see $err"
+	fi
+'
+
+# three-tree test
+
+test_expect_success 'three-way not complaining' '
+
+	rm -f file2 &&
+	git checkout side &&
+	echo >file3 file three &&
+	git update-index --add file3 &&
+	git commit -a -m "side adds file3" &&
+
+	git checkout master &&
+	echo >file2 file two is untracked on the master side &&
+
+	git-read-tree -m -u branch-point master side
+'
+
+test_done

unpack-trees.c

@@ -642,7 +642,7 @@ int threeway_merge(struct cache_entry **stages,
 		    (remote_deleted && head && head_match)) {
 			if (index)
 				return deleted_entry(index, index, o);
-			else if (path)
+			else if (path && !head_deleted)
 				verify_absent(path, "removed", o);
 			return 0;
 		}
@@ -661,8 +661,6 @@ int threeway_merge(struct cache_entry **stages,
 	if (index) {
 		verify_uptodate(index, o);
 	}
-	else if (path)
-		verify_absent(path, "overwritten", o);
 
 	o->nontrivial_merge = 1;