Git: list all commits on all remote branches?

Try this -

git log --branches=*

The following should do what you want:

git rev-list --remotes

The documentation for the --remotes option says:

--remotes[=<pattern>]

Pretend as if all the refs in refs/remotes are listed on the command line as <commit>. If <pattern> is given, limit remote-tracking branches to ones matching given shell glob. If pattern lacks ?, , or [, / at the end is implied.

As a point of pedantry, these are "remote-tracking branches" rather than "remote branches" - the latter is a somewhat ambiguous term in git, since you might be referring to a branch in remote repository or a remote-tracking branch (which is like a cache of the state of the branch in the remote repository).