I reran the test with another instance of the same debug executable but with the './emacs -Q' option. Similar steps as last time--I typed 'M-x magit' and pressed 'l' to enter the log buffer.

This time it failed at another assertion:

"insdel.c:295: Emacs fatal error: assertion failed: m->bytepos >= m->charpos && m->bytepos - m->charpos <= Z_BYTE - Z"

New debug backtrace is attached.