Updated git-subtree.sh to handle annotated tags

[amzz]

Command was using tag SHA1 key although it should dereference the tag for certain commit before running the 'git subtree pull'.

Below example that triggered the issue

Pull older version of ofc_lib

git subtree pull -P plesiosync_lib/ip/ofc_lib ofc_lib ofc_lib-v0.3 --squash -d

Hack sub-project in super-project

xemacs -nw plesiosync_lib/ip/ofc_lib/src/overflowcontrol.vhdl
git commit -am "testing hack"

Pull latest ofc-lib

git subtree pull -P plesiosync_lib/ip/ofc_lib ofc_lib ofc_lib-v0.7 --squash -d

Resolve conflict

git mergetool
git commit

Check the commit id for ofc_lib

git subtree split -P plesiosync_lib/ip/ofc_lib
fatal: 7e5957c2b61a46221b9c02c43e1f9c3925eacb76 is not a valid 'commit' object
Can't copy commit 2ff99337166eb08db236828cce4384430390a37c

The hash key in fatal line is the annotateg tag key and not the actual commit key.