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rebase: change the default backend from "am" to "merge"

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The am-backend drops information and thus limits what we can do:

  * lack of full tree information from the original commits means we
    cannot do directory rename detection and warn users that they might
    want to move some of their new files that they placed in old
    directories to prevent their becoming orphaned.[1]
  * reduction in context from only having a few lines beyond those
    changed means that when context lines are non-unique we can apply
    patches incorrectly.[2]
  * lack of access to original commits means that conflict marker
    annotation has less information available.
  * the am backend has safety problems with an ill-timed interrupt.

Also, the merge/interactive backend have far more abilities, appear to
currently have a slight performance advantage[3] and have room for more
optimizations than the am backend[4] (and work is underway to take
advantage of some of those possibilities).

[1] https://lore.kernel.org/git/xmqqh8jeh1id.fsf@gitster-ct.c.googlers.com/
[2] https://lore.kernel.org/git/CABPp-BGiu2nVMQY_t-rnFR5GQUz_ipyEE8oDocKeO+h+t4Mn4A@mail.gmail.com/
[3] https://public-inbox.org/git/CABPp-BF=ev03WgODk6TMQmuNoatg2kiEe5DR__gJ0OTVqHSnfQ@mail.gmail.com/
[4] https://lore.kernel.org/git/CABPp-BGh7yW69QwxQb13K0HM38NKmQif3A6C6UULEKYnkEJ5vA@mail.gmail.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Elijah Newren
2020-02-15 21:36:40 +00:00
committed by Junio C Hamano
parent 8295ed690b
commit 2ac0d6273f
4 changed files with 22 additions and 8 deletions
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4
builtin/rebase.c
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@ -101,7 +101,7 @@ struct rebase_options {
#define REBASE_OPTIONS_INIT { \
.type = REBASE_UNSPECIFIED, \
.empty = EMPTY_UNSPECIFIED, \
.default_backend = "am", \
.default_backend = "merge", \
.flags = REBASE_NO_QUIET, \
.git_am_opts = ARGV_ARRAY_INIT, \
.git_format_patch_opt = STRBUF_INIT \
@ -1917,7 +1917,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
if (options.type == REBASE_UNSPECIFIED) {
if (!strcmp(options.default_backend, "merge"))
options.type = REBASE_MERGE;
imply_interactive(&options, "--merge");
else if (!strcmp(options.default_backend, "am"))
options.type = REBASE_AM;
else