Browse orphaned commits in Git

Rather than leave this open I think I'll give an answer to my own question. Using git reflog --all is a good way to browse orphaned commits - and using the SHA1 hashes from that you can reconstruct history.

In my case though, the repository was corrupted so this didn't help; git fsck can help you find and sometimes fix errors in the repository itself.

With git 2.9.x/2.10 (Q3 2016), you won't have to use git reflog --all anymore, git reflog will be enough.

See commit 71abeb7 (03 Jun 2016) by SZEDER Gábor (szeder).
^{(Merged by Junio C Hamano -- gitster -- in commit 7949837, 06 Jul 2016)}

reflog: continue walking the reflog past root commits

If a repository contains more than one root commit, then its HEAD reflog may contain multiple "creation events", i.e. entries whose "from" value is the null sha1.
Listing such a reflog currently stops prematurely at the first such entry, even when the reflog still contains older entries.
This can scare users into thinking that their reflog got truncated after 'git checkout --orphan'.

Continue walking the reflog past such creation events based on the preceeding reflog entry's "new" value.

I typically find the git reflog output to be confusing. It's much easier for me to understand a commit graph from git log --graph --reflog. Overriding the log format to show only commit summaries also can make the graph easier to follow:

$ git config --global alias.graph \
    "log --graph --all --format='%h %s%n        (%an, %ar)%d' --abbrev-commit"

$ git graph --reflog

* f06abeb Add feature
|         (Sue Dakota, 4 days ago) (HEAD -> master)
* f126291 Fix the build
|         (Oski M. Wizard, 5 days ago) (origin/master, master)
* 3c4fb9c Move fast, break things
|         (Alyssa P. Hacker, 5 days ago)
| * e3124bf fixup! More work for feature
| |         (Sue Dakota, 4 days ago)
| | * 6a7a52e Lost commit
| |/          (Sue Dakota, 4 days ago)
| * 69d9438 More work for feature
| |         (Sue Dakota, 2 weeks ago)
| * 8f69aba Initial work for feature
|/          (Sue Dakota, 3 weeks ago)
* d824fa9 Fix warnings from the linter
|         (Theo Rhys Tudent, 4 weeks ago)
* 9f782b8 Fix test flakes
|         (Tess Driven, 5 weeks ago)

From that it's clear that e3124bf and 6a7a52e are unreferenced orphans, and there's context from their ancestor commits.