build: don't fail if given a git submodule which does not exist

If going back in time in git history, across a commit that introduces a new
submodule, the 'git-submodule.sh' script will fail, causing rebuild to fail.

This is because config-host.mak contains a GIT_SUBMODULES variable that lists
a submodule that only exists in the later commit. config-host.mak won't get
repopulated until config.status is invoked, but make won't get this far due to
the submodule error.

This change makes 'git-submodule.sh' check whether each module is known to git
and drops any which are not present. A warning message will be printed when any
submodule is dropped in this manner.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

scripts/git-submodule.sh

@@ -7,7 +7,7 @@ substat=".git-submodule-status"
 
 command=$1
 shift
-modules="$@"
+maybe_modules="$@"
 
 test -z "$GIT" && GIT=git
 
@@ -33,12 +33,24 @@ error() {
     exit 1
 }
 
-if test -z "$modules"
+if test -z "$maybe_modules"
 then
     test -e $substat || touch $substat
     exit 0
 fi
 
+modules=""
+for m in $maybe_modules
+do
+    $GIT submodule status $m 1> /dev/null 2>&1
+    if test $? = 0
+    then
+        modules="$modules $m"
+    else
+        echo "warn: ignoring non-existent submodule $m"
+    fi
+done
+
 if ! test -e ".git"
 then
     echo "$0: unexpectedly called with submodules but no git checkout exists"