t6000lib: workaround a possible dash bug

pdksh doesn't need this patch, of course bash works fine since
that what most users use.

Normally, 'var=val command' seems to work fine with dash, but
perhaps there's something weird going on with "$@".  dash is
pretty widespread, so it'll be good to support this even though
it does seem like a bug in dash.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t6000lib.sh

@@ -69,7 +69,9 @@ on_committer_date()
 {
     _date=$1
     shift 1
-    GIT_COMMITTER_DATE=$_date "$@"
+    export GIT_COMMITTER_DATE="$_date"
+    "$@"
+    unset GIT_COMMITTER_DATE
 }
 
 # Execute a command and suppress any error output.