[PATCH] Prevent force updating of the current branch

I noticed when trying to improve the user-experience of creating ambiguous branches [1] that there are insufficient checks to safeguard git-branch -M and git-checkout -B from over-writing the current branch.

Conrad

[1] http://thread.gmane.org/gmane.comp.version-control.git/179503

---
 branch.c                   |   34 ++++++++++++++++++++++++----------
 branch.h                   |    8 ++++++++
 builtin/branch.c           |    6 +-----
 builtin/checkout.c         |   12 +++---------
 t/t2018-checkout-branch.sh |   17 +++++++++++++++++
 t/t3200-branch.sh          |   12 ++++++++++++
 6 files changed, 65 insertions(+), 24 deletions(-)