From: Zach FettersMoore <zach.fetters@apollographql.com>

When there are multiple subtrees present in a repository and they are
all using 'git subtree split', the 'split' command can take a
significant (and constantly growing) amount of time to run even when
using the '--rejoin' flag. This is due to the fact that when processing
commits to determine the last known split to start from when looking
for changes, if there has been a split/merge done from another subtree
there will be 2 split commits, one mainline and one subtree, for the
second subtree that are part of the processing. The non-mainline
subtree split commit will cause the processing to always need to search
the entire history of the given subtree as part of its processing even
though those commits are totally irrelevant to the current subtree
split being run.

In the diagram below, 'M' represents the mainline repo branch, 'A'
represents one subtree, and 'B' represents another. M3 and B1 represent
a split commit for subtree B that was created from commit M4. M2 and A1
represent a split commit made from subtree A that was also created
based on changes back to and including M4. M1 represents new changes to
the repo, in this scenario if you try to run a 'git subtree split
--rejoin' for subtree B, commits M1, M2, and A1, will be included in
the processing of changes for the new split commit since the last
split/rejoin for subtree B was at M3. The issue is that by having A1
included in this processing the command ends up needing to processing
every commit down tree A even though none of that is needed or relevant
to the current command and result.

M1
 |	  \	  \
M2	   |	   |
 |     	  A1	   |
M3	   |	   |
 |	   |	  B1
M4	   |	   |

So this commit makes a change to the processing of commits for the split
command in order to ignore non-mainline commits from other subtrees such
as A1 in the diagram by adding a new function
'should_ignore_subtree_commit' which is called during
'process_split_commit'. This allows the split/rejoin processing to still
function as expected but removes all of the unnecessary processing that
takes place currently which greatly inflates the processing time.

Added a test to validate that the proposed fix
solves the issue.

The test accomplishes this by checking the output
of the split command to ensure the output from
the progress of 'process_split_commit' function
that represents the 'extracount' of commits
processed does not increment.

This was tested against the original functionality
to show the test failed, and then with this fix
to show the test passes.

This illustrated that when using multiple subtrees,
A and B, when doing a split on subtree B, the
processing does not traverse the entire history
of subtree A which is unnecessary and would cause
the 'extracount' of processed commits to climb
based on the number of commits in the history of
subtree A.

Signed-off-by: Zach FettersMoore <zach.fetters@apollographql.com>

Submitted-As: https://lore.kernel.org/git/pull.1587.v4.git.1698347871200.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1587.git.1695067516192.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1587.v2.git.1695399920.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1587.v3.git.1696019580.gitgitgadget@gmail.com