[PATCH v2 0/3] teach git-checkout -B WAS builtin/checkout: DWIM with -f -b

Right now, the -f/--force in git-checkout doesn't carry over to -b
operations - ie. you can't do git checkout -b branch on an existing
branch.

This patch allows you to do this with the -B option, so that one can do

  git checkout -B branch ref

which is equivalent to

  git branch -f branch ref && git checkout branch

Changes from v1:
 - reworked option handling
 - don't replace <new_branch> with <branch> in documentation

Contents:
[PATCH v2 1/3] add tests for checkout -b
[PATCH v2 2/3] builtin/checkout: reword hint for -b
[PATCH v2 3/3] builtin/checkout: learn -B

 Documentation/git-checkout.txt |   21 +++++-
 builtin/checkout.c             |   32 ++++++--
 t/t2018-checkout-branch.sh     |  166 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 211 insertions(+), 8 deletions(-)
 create mode 100755 t/t2018-checkout-branch.sh