reset.c: extract function for updating {ORIG_,}HEAD
reset.c: extract function for updating {ORIG_,}HEAD

By extracting the code for updating the HEAD and ORIG_HEAD symbolic
references to a separate function, we declutter cmd_reset() a bit and
we make it clear that e.g. the four variables {,sha1_}{,old_}orig are
only used by this code.

Signed-off-by: Martin von Zweigbergk <efe9a872bcf008f3e03ada5660d40f3164417d62@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git