Have you tried?
$ git log --full-history {path/to/file}
...
commit 5574c123456c1c60e87fa072ea9cbe56ffe34a0
Merge: d262137 be9c501
Author: {author}
Date: Tue Jun 18 09:37:17 2013 -0400
... more commits ...
This shows a pretty good summary of the information you're likely looking for. In this example, we get the commit hash of the latest commits and if it is a merged commit, it will show both sides of the merge. To compare these, you could use a command like the following:
$ git diff d262137..be9c501 -- {path/to/file}
If you don't have a merge commit, you can simply git diff
the latest commit hashes to see the differences.