How to abort 'git commit --amend'?

If you delete the commit message (no need to delete the ones starting with #) you will abort the git commit --amend command. You will get output like this:

Aborting commit due to empty commit message.

That is correct. Saving the file with no contents will abort the amend.

Adding another answer to this, you can also do

:cq

to abort the amend in Vim which tells it to quit with an error code, causing Git to abort with:

error: There was a problem with the editor 'Vim'.
Please supply the message using either -m or -F option.

From Vim's documentation:

:cq[uit][!]             Quit Vim with an error code, so that the compiler
                        will not compile the same file again.
                        WARNING: All changes in files are lost!  Also when the
                        [!] is not used.  It works like ":qall!" :qall,
                        except that Vim returns a non-zero exit code.

delete the message. an empty message will abort any commit (amend is just a 'special' commit)