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

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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
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4
builtin/merge.c
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@ -87,6 +87,7 @@ static int signoff;
static const char *sign_commit;
static int autostash;
static int no_verify;
static char *into_name;
static struct strategy all_strategy[] = {
{ "recursive", NO_TRIVIAL },
@ -286,6 +287,8 @@ static struct option builtin_merge_options[] = {
{ OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
N_("read message from file"), PARSE_OPT_NONEG,
NULL, 0, option_read_message },
OPT_STRING(0, "into-name", &into_name, N_("name"),
N_("use <name> instead of the real target")),
OPT__VERBOSITY(&verbosity),
OPT_BOOL(0, "abort", &abort_current_merge,
N_("abort the current in-progress merge")),
@ -1122,6 +1125,7 @@ static void prepare_merge_message(struct strbuf *merge_names, struct strbuf *mer
opts.add_title = !have_message;
opts.shortlog_len = shortlog_len;
opts.credit_people = (0 < option_edit);
opts.into_name = into_name;
fmt_merge_msg(merge_names, merge_msg, &opts);
if (merge_msg->len)