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If you use '--squash', the merge direction doesn't always have to be

forward; you can use this command to go back in time from v2.5 to v2.4,

for example. If your merge introduces a conflict, you can resolve it in

the usual ways.

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After splitting successfully, a single commit id is printed to stdout.

This corresponds to the HEAD of the newly created tree, which you can

manipulate however you want.

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Repeated splits of exactly the same history are guaranteed to be

identical (i.e. to produce the same commit ids). Because of this, if

you add new commits and then re-split, the new commits will be attached

as commits on top of the history you generated last time, so 'git merge'

and friends will work as expected.

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Note that if you use '--squash' when you merge, you should usually not

just '--rejoin' when you split.

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Instead of merging the entire history from the subtree project, produce

only a single commit that contains all the differences you want to

merge, and then merge that new commit into your project.

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Using this option helps to reduce log clutter. People rarely want to see

every change that happened between v1.0 and v1.1 of the library they're

using, since none of the interim versions were ever included in their

application.

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Using '--squash' also helps avoid problems when the same subproject is

included multiple times in the same project, or is removed and then

re-added. In such a case, it doesn't make sense to combine the

histories anyway, since it's unclear which part of the history belongs

to which subtree.

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Furthermore, with '--squash', you can switch back and forth between

different versions of a subtree, rather than strictly forward. 'git

subtree merge --squash' always adjusts the subtree to match the exactly

specified commit, even if getting to that commit would require undoing

some changes that were added earlier.

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Whether or not you use '--squash', changes made in your local repository

remain intact and can be later split and send upstream to the

subproject.

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When generating synthetic history, add <annotation> as a prefix to each

commit message. Since we're creating new commits with the same commit

message, but possibly different content, from the original commits, this

can help to differentiate them and avoid confusion.

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Whenever you split, you need to use the same <annotation>, or else you

don't have a guarantee that the new re-created history will be identical

to the old one. That will prevent merging from working correctly. git

subtree tries to make it work anyway, particularly if you use --rejoin,

but it may not always be effective.

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After generating the synthetic history, create a new branch called

<branch> that contains the new history. This is suitable for immediate

pushing upstream. <branch> must not already exist.

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If you use '--rejoin', git subtree attempts to optimize its history

reconstruction to generate only the new commits since the last

'--rejoin'. '--ignore-join' disables this behaviour, forcing it to

regenerate the entire history. In a large project, this can take a long

time.

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If your subtree was originally imported using something other than git

subtree, its history may not match what git subtree is expecting. In

that case, you can specify the commit id <onto> that corresponds to the

first revision of the subproject's history that was imported into your

project, and git subtree will attempt to build its history from there.

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If you used 'git subtree add', you should never need this option.

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After splitting, merge the newly created synthetic history back into

your main project. That way, future splits can search only the part of

history that has been added since the most recent --rejoin.

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If your split commits end up merged into the upstream subproject, and

then you want to get the latest upstream version, this will allow git's

merge algorithm to more intelligently avoid conflicts (since it knows

these synthetic commits are already part of the upstream repository).

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Unfortunately, using this option results in 'git log' showing an extra

copy of every new commit that was created (the original, and the

synthetic one).

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If you do all your merges with '--squash', don't use '--rejoin' when you

split, because you don't want the subproject's history to be part of

your project anyway.