We now keep track of 3 sets during traversal:
- keep: commits we've reached from head commits
- drop: commits we've reached from tail commits
- skip: ancestors of commits in both 'keep' and 'drop'
Commits in 'keep' and/or 'drop' may be added later to the 'skip' set
if we discover later that they are part of a common subgraph of the
head and tail commits.
From these sets we can calculate the commits we are interested in:
lca commits are those in 'keep' and 'drop', but not in 'skip'.
findtwixt commits are those in 'keep', but not in 'drop' or 'skip'.
The "LCA" commit returned is a common ancestor such that there are no
other common ancestors that can reach that commit. Although there can
be multiple commits that meet this criteria, where one is technically
lower on the commit-graph than the other, these cases only happen in
complex merge arrangements and any choice is likely a decent merge
base.
Repainting is now done in paint() directly. When we find a boundary
commit, we switch our paint color to 'skip'. 'skip' painting does
not stop when it hits another color; we continue until we are left
with only 'skip' commits on the queue.
This fixes several mishandled cases in the current algorithm:
1. If we hit the common subgraph from tail commits first (if the tail
commit was newer than the head commit), we ended up traversing the
entire commit graph. This is because we couldn't distinguish
between 'drop' commits that were part of the common subgraph, and
those that were still looking for it.
2. If we traversed through an initial part of the common subgraph from
head commits before reaching it from tail commits, these commits
were returned from findtwixt even though they were also reachable
from tail commits.
3. In the same case as 2, we might end up choosing an incorrect
commit as the LCA, which is an ancestor of the real LCA.
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