Meaning of two commit-ish hash in git diff

From: Yubin Ruan <ablacktshirt@gmail.com>
Date: Oct 31, 2017, 03:24 UTC
Message-ID: <ab9b5c63-ca38-6157-6732-78aa5d5f291f@gmail.com>

Hi,

Can anyone tell me what does the "f8886b4..a1c96df" mean in a git diff output, as below?

diff --git a/path/somefile b/path/somefile
index f8886b4..a1c96df 100644
--- a/path/somefile
+++ b/path/somefile
<snip>

This is output by a `git diff` between two adjacent commits but they are
not any commit hash. I grep through the whole $(git log) but still cannot
find those hash.

And BTW, are there any special meanings for that .. ?

Thanks,
Yubin