Re: git merge deletes my changes

["Jakub Narębski" <jnareb@gmail.com>]

W dniu 10.10.2016 o 19:52, Paul Smith pisze:
> On Mon, 2016-10-10 at 10:19 +0000, Eduard Egorov wrote:
>> # ~/gitbuild/git-2.10.1/git merge -s subtree --squash ceph_ansible
>>
>> Can somebody confirm this please? Doesn't "merge -s subtree" really
>> merges branches?
> 
> I think possibly you're not fully understanding what the --squash flag
> does... that's what's causing your issue here, not the "-s" option.
> 
> A squash merge takes the commits that would be merged from the origin
> branch and squashes them into a single patch and applies them to the
> current branch as a new commit... but this new commit is not a merge
> commit (that is, when you look at it with "git show" etc. the commit
> will have only one parent, not two--or more--parents like a normal merge
> commit).
> 
> Basically, it's syntactic sugar for a diff plus patch operation plus
> some Git goodness wrapped around it to make it easier to use.

Actually this is full merge + commit surgery (as if you did merge with
--no-commit, then deleted MERGE_HEAD); the state of worktree is as if
it were after a merge.

> 
> But ultimately once you're done, Git has no idea that this new commit
> has any relationship whatsoever to the origin branch.  So the next time
> you merge, Git doesn't know that there was a previous merge and it will
> try to merge everything from scratch rather than starting at the
> previous common merge point.
> 
> So either you'll have to use a normal, non-squash merge, or else you'll
> have to tell Git by hand what the previous common merge point was (as
> Jeff King's excellent email suggests).  Or else, you'll have to live
> with this behavior.

The `git subtree` command (from contrib) allows yet another way: it
squashes *history* of merged subproject (as if with interactive rebase
'squash'), then merges this squash commit.

Now I know why this feature is here...
-- 
Jakub Narębski