Git history flattener https://twitter.com/mild_sunrise/status/1300181598306996224 (needs GitPython)

flatten.py

#!/usr/bin/env python3
from git import Repo, Commit
list_commits = lambda commits: '\n'.join('  {}  {}'.format(n.hexsha[:8], n.message.splitlines()[0]) for n in commits)

repo = Repo.init('path to bare clone', bare=True, mkdir=False)
refs = repo.refs

# load whole graph
def visit_graph(roots, get_parents=lambda n: n.parents):
    queue = list(roots)
    graph = set()
    while queue:
        n = queue.pop(0)
        graph.add(n)
        for parent in get_parents(n):
            if not (parent in graph or parent in queue):
                queue.append(parent)
    return graph

refs = { r: r.commit for r in refs }
graph = { n: { "parents": list(n.parents) } for n in visit_graph(refs.values()) }

# processing step (squash merge commits that point to same tree as one parent)
def squash_into(c1, c2):
    # update pointers to c1, so they point to c2
    for desc in graph.values():
        for i, parent in enumerate(desc['parents']):
            if parent == c1:
                desc['parents'][i] = c2
    for k, target in refs.items():
        if target == c1:
            refs[k] = c2
    # remove c1 from graph
    del graph[c1]

total = 0
squashed = []
for n in list(graph):
    if len(n.parents) > 1:
        total += 1
        candidates = [ parent for parent in graph[n]['parents'] if parent.tree == n.tree ]
        if candidates:
            squash_into(n, candidates[0])
            squashed.append(n)
print('{} of {} merge commits were squashed\n{}\n'.format(len(squashed), total, list_commits(squashed)))

# check for additional orphaned commits
orphans = set(graph) - visit_graph(refs.values(), lambda n: graph[n]['parents'])
print('{} additional commits were lost (due to changes being in both branches)\n{}\n'.format(len(orphans), list_commits(orphans)))

# rewrite step (emit rewritten commits)
def rewrite(n):
    if 'rewritten' not in graph[n]:
        parents = [rewrite(c) for c in graph[n]['parents']]
        n2 = Commit.create_from_tree(repo=n.repo, tree=n.tree, message=n.message.encode(n.encoding),
                                     parent_commits=parents, author=n.author, committer=n.committer,
                                     author_date=n.authored_datetime, commit_date=n.committed_datetime)
        graph[n]['rewritten'] = n2
    return graph[n]['rewritten']
for n in graph:
    rewrite(n)

# point refs to rewritten commits
for r, target in refs.items():
    if r.is_detached and not r.is_remote():
        r.set_commit(graph[target]['rewritten'])

print('done!')