Merge branch 'nm/submodule-update-force'

* nm/submodule-update-force:
  submodule: Add --force option for git submodule update

Conflicts:
	t/t7406-submodule-update.sh

Documentation/git-submodule.txt

@@ -186,8 +186,10 @@ OPTIONS
 
 -f::
 --force::
-	This option is only valid for the add command.
-	Allow adding an otherwise ignored submodule path.
+	This option is only valid for add and update commands.
+	When running add, allow adding an otherwise ignored submodule path.
+	When running update, throw away local changes in submodules when
+	switching to a different commit.
 
 --cached::
 	This option is only valid for status and summary commands.  These

git-submodule.sh

@@ -8,7 +8,7 @@ dashless=$(basename "$0" | sed -e 's/-/ /')
 USAGE="[--quiet] add [-b branch] [-f|--force] [--reference <repository>] [--] <repository> [<path>]
    or: $dashless [--quiet] status [--cached] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] init [--] [<path>...]
-   or: $dashless [--quiet] update [--init] [-N|--no-fetch] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
+   or: $dashless [--quiet] update [--init] [-N|--no-fetch] [-f|--force] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
    or: $dashless [--quiet] foreach [--recursive] <command>
    or: $dashless [--quiet] sync [--] [<path>...]"
@@ -402,6 +402,9 @@ cmd_update()
 		-N|--no-fetch)
 			nofetch=1
 			;;
+		-f|--force)
+			force=$1
+			;;
 		-r|--rebase)
 			update="rebase"
 			;;
@@ -480,10 +483,11 @@ cmd_update()
 
 		if test "$subsha1" != "$sha1"
 		then
-			force=
-			if test -z "$subsha1"
+			subforce=$force
+			# If we don't already have a -f flag and the submodule has never been checked out
+			if test -z "$subsha1" -a -z "$force"
 			then
-				force="-f"
+				subforce="-f"
 			fi
 
 			if test -z "$nofetch"
@@ -515,7 +519,7 @@ cmd_update()
 				msg="merged in"
 				;;
 			*)
-				command="git checkout $force -q"
+				command="git checkout $subforce -q"
 				action="checkout"
 				msg="checked out"
 				;;

t/t7406-submodule-update.sh

@@ -94,6 +94,29 @@ test_expect_success 'submodule update does not fetch already present commits' '
 	! test -s actual.err
 '
 
+test_expect_success 'submodule update should fail due to local changes' '
+	(cd super/submodule &&
+	 git reset --hard HEAD~1 &&
+	 echo "local change" > file
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 test_must_fail git submodule update submodule
+	)
+'
+test_expect_success 'submodule update should throw away changes with --force ' '
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --force submodule &&
+	 cd submodule &&
+	 ! compare_head
+	)
+'
+
 test_expect_success 'submodule update --rebase staying on master' '
 	(cd super/submodule &&
 	  git checkout master