git - fatal: Unknown index entry format

When your index is broken you can normally delete the index file and reset it.

rm -f .git/index
git reset

or you clone the repo again.

Regardless of the error code at the end of

fatal: Unknown index entry format

your git index file is corrupt. (I achieved this error after doing a find and replace for all files.)

If using a Windows Command Prompt,

del .git\index

will delete the index file and

git reset

will reset it.