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>