allow checkout and branch to create branches on a merge base

Thanks again for the review, Junio.

I've squashed 2/3 and 3/3 together and made that documentation change I was talking about earlier.

---

Changes since v2:

* Squashed 2/3 with 3/3
* Document merge base syntax for <start_point> in git-checkout.txt

Changes since v1:

* Moved multiple test_when_finished calls that appeared in "reverse order" into one call that appears in the logical order
* Made create_branch handle merge base revs instead of putting a hack into checkout

Denton Liu (2):
t2018: cleanup in current test
branch: make create_branch accept a merge base rev

 Documentation/git-branch.txt   |  6 +++-
 Documentation/git-checkout.txt |  4 +++
 branch.c                       |  2 +-
 t/t2018-checkout-branch.sh     | 56 ++++++++++++++++++----------------
 t/t3200-branch.sh              | 14 ++++++---
 5 files changed, 50 insertions(+), 32 deletions(-)

Range-diff against v2:

1:  9d04faf29d = 1:  9d04faf29d t2018: cleanup in current test
2:  5e8320cd80 < -:  ---------- t2018: demonstrate checkout -b merge base bug
3:  c91c7535a7 ! 2:  bb25852740 branch: make create_branch accept a merge base rev

@@ -41,6 +41,21 @@
Note that this will create the new branch, but it will not switch the working tree to it; use "git checkout <newbranch>" to switch to the

+ diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
+ --- a/Documentation/git-checkout.txt
+ +++ b/Documentation/git-checkout.txt
+@@
+ <start_point>::
+ The name of a commit at which to start the new branch; see
+ linkgit:git-branch[1] for details. Defaults to HEAD.
+++
++As a special case, you may use "A...B" as a shortcut for the ++merge base of A and B if there is exactly one merge base. You can ++leave out at most one of A and B, in which case it defaults to HEAD. +
+ <tree-ish>::
+ Tree to checkout from (when paths are given). If not specified,
+
diff --git a/branch.c b/branch.c
--- a/branch.c
+++ b/branch.c
@@ -61,20 +76,29 @@
do_checkout branch2
'

--test_expect_failure 'checkout -b to a merge base' '
+test_expect_success 'checkout -b to a merge base' '
++ test_when_finished "
++ git checkout branch1 &&
++ test_might_fail git branch -D branch2" &&
++ git checkout -b branch2 branch1...
++'
++
+ test_expect_success 'checkout -b to a new branch, set to an explicit ref' ' test_when_finished "
git checkout branch1 &&
- test_might_fail git branch -D branch2" &&
@@
do_checkout branch2 "" -B
'

--test_expect_failure 'checkout -B to a merge base' '
+test_expect_success 'checkout -B to a merge base' '
- git checkout branch1 &&
++ git checkout branch1 &&
++
++ git checkout -B branch2 branch1...
++'
++
+ test_expect_success 'checkout -B to an existing branch from detached HEAD resets branch to HEAD' '
+ git checkout $(git rev-parse --verify HEAD) &&

- git checkout -B branch2 branch1...

diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh --- a/t/t3200-branch.sh

2.21.0.1000.g11cd861522