merge: allow to pretend a merge is made into a different branch

When a series of patches for a topic-B depends on having topic-A,
the workflow to prepare the topic-B branch would look like this:

    $ git checkout -b topic-B main
    $ git merge --no-ff --no-edit topic-A
    $ git am <mbox-for-topic-B

When topic-A gets updated, recreating the first merge and rebasing
the rest of the topic-B, all on detached HEAD, is a useful
technique.  After updating topic-A with its new round of patches:

    $ git checkout topic-B
    $ prev=$(git rev-parse 'HEAD^{/^Merge branch .topic-A. into}')
    $ git checkout --detach $prev^1
    $ git merge --no-ff --no-edit topic-A
    $ git rebase --onto HEAD $prev @{-1}^0
    $ git checkout -B @{-1}

This will

 (0) check out the current topic-B.
 (1) find the previous merge of topic-A into topic-B.
 (2) detach the HEAD to the parent of the previous merge.
 (3) merge the updated topic-A to it.
 (4) reapply the patches to rebuild the rest of topic-B.
 (5) update topic-B with the result.

without contaminating the reflog of topic-B too much.  topic-B@{1}
is the "logically previous" state before topic-A got updated, for
example.  At (4), comparison (e.g. range-diff) between HEAD and
@{-1} is a meaningful way to sanity check the result, and the same
can be done at (5) by comparing topic-B and topic-B@{1}.

But there is one glitch.  The merge into the detached HEAD done in
the step (3) above gives us "Merge branch 'topic-A' into HEAD", and
does not say "into topic-B".

Teach the "--into-name=<branch>" option to "git merge" and its
underlying "git fmt-merge-message", to pretend as if we were merging
into <branch>, no matter what branch we are actually merging into,
when they prepare the merge message.  The pretend name honors the
usual "into <target>" suppression mechanism, which can be seen in
the tests added here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Junio C Hamano
2021-12-20 14:53:43 -08:00
parent cd3e606211
commit bd2bc94252
7 changed files with 67 additions and 5 deletions

View File

@ -573,7 +573,35 @@ test_expect_success 'merge-msg with "merging" an annotated tag' '
test_cmp expected .git/MERGE_MSG
'
test_expect_success 'merge --into-name=<name>' '
test_when_finished "git checkout main" &&
git checkout -B side main &&
git commit --allow-empty -m "One step ahead" &&
git checkout --detach main &&
git merge --no-ff side &&
git show -s --format="%s" >full.0 &&
head -n1 full.0 >actual &&
# expect that HEAD is shown as-is
grep -e "Merge branch .side. into HEAD$" actual &&
git reset --hard main &&
git merge --no-ff --into-name=main side &&
git show -s --format="%s" >full.1 &&
head -n1 full.1 >actual &&
# expect that we pretend to be merging to main, that is suppressed
grep -e "Merge branch .side.$" actual &&
git checkout -b throwaway main &&
git merge --no-ff --into-name=main side &&
git show -s --format="%s" >full.2 &&
head -n1 full.2 >actual &&
# expect that we pretend to be merging to main, that is suppressed
grep -e "Merge branch .side.$" actual
'
test_expect_success 'merge.suppressDest configuration' '
test_when_finished "git checkout main" &&
git checkout -B side main &&
git commit --allow-empty -m "One step ahead" &&
git checkout main &&
@ -590,7 +618,19 @@ test_expect_success 'merge.suppressDest configuration' '
git -c merge.suppressDest="ma?*[rn]" fmt-merge-msg <.git/FETCH_HEAD >full.3 &&
head -n1 full.3 >actual &&
grep -e "Merge branch .side." actual &&
! grep -e " into main$" actual
! grep -e " into main$" actual &&
git checkout --detach HEAD &&
git -c merge.suppressDest="main" fmt-merge-msg <.git/FETCH_HEAD >full.4 &&
head -n1 full.4 >actual &&
grep -e "Merge branch .side. into HEAD$" actual &&
git -c merge.suppressDest="main" fmt-merge-msg \
--into-name=main <.git/FETCH_HEAD >full.5 &&
head -n1 full.5 >actual &&
grep -e "Merge branch .side." actual &&
! grep -e " into main$" actual &&
! grep -e " into HEAD$" actual
'
test_done