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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/fmt-merge-msg.c
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@ -12,6 +12,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
{
const char *inpath = NULL;
const char *message = NULL;
char *into_name = NULL;
int shortlog_len = -1;
struct option options[] = {
{ OPTION_INTEGER, 0, "log", &shortlog_len, N_("n"),
@ -23,6 +24,8 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
DEFAULT_MERGE_LOG_LEN },
OPT_STRING('m', "message", &message, N_("text"),
N_("use <text> as start of message")),
OPT_STRING(0, "into-name", &into_name, N_("name"),
N_("use <name> instead of the real target branch")),
OPT_FILENAME('F', "file", &inpath, N_("file to read from")),
OPT_END()
};
@ -56,6 +59,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
opts.add_title = !message;
opts.credit_people = 1;
opts.shortlog_len = shortlog_len;
opts.into_name = into_name;
ret = fmt_merge_msg(&input, &output, &opts);
if (ret)