Fix migration script to use original SHA1 from tempest's commits

Use git log instead of git filter-branch - which rewrites the history - to get
SHA1 from the commits that touch the files to be migrated from tempest to
tempest_lib.

Change-Id: I321d78390991eb90fbb4a176a12187b78edc9f31
Closes-bug: #1386782
Merge "Fix migration script to use original SHA1 from tempest's commits"