[PATCH] fix null TZ problems on os/x

It seems that the localtime() libc routine doesn't care for finding a TZ
that's empty.  It's ok with TZ not being set.  Setting the TZ to GMT allowed
these tests to pass.

$ uname -v
Darwin Kernel Version 7.9.0: Wed Mar 30 20:11:17 PST 2005; root:xnu/xnu-517.12.7.obj~1/RELEASE_PPC
Signed-off-by: NBrad Roberts <braddr@puremagic.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t1100-commit-tree-options.sh

@@ -32,7 +32,7 @@ test_expect_success \
      GIT_COMMITTER_NAME="Committer Name" \
      GIT_COMMITTER_EMAIL="committer@email" \
      GIT_COMMITTER_DATE="2005-05-26 23:30" \
-     TZ= git-commit-tree `cat treeid` >commitid 2>/dev/null'
+     TZ=GMT git-commit-tree `cat treeid` >commitid 2>/dev/null'
 
 test_expect_success \
     'read commit' \

t/t5000-tar-tree.sh

@@ -41,7 +41,7 @@ test_expect_success \
      find a -type l | xargs git-update-cache --add &&
      treeid=`git-write-tree` &&
      echo $treeid >treeid &&
-     TZ= GIT_COMMITTER_DATE="2005-05-27 22:00:00" \
+     TZ=GMT GIT_COMMITTER_DATE="2005-05-27 22:00:00" \
      git-commit-tree $treeid </dev/null >.git/HEAD'
 
 test_expect_success \
@@ -50,7 +50,7 @@ test_expect_success \
 
 test_expect_success \
     'validate file modification time' \
-    'TZ= tar tvf b.tar a/a |
+    'TZ=GMT tar tvf b.tar a/a |
      awk \{print\ \$4,\ \(length\(\$5\)\<7\)\ ?\ \$5\":00\"\ :\ \$5\} \
      >b.mtime &&
      echo "2005-05-27 22:00:00" >expected.mtime &&