git diff file against its last change

This does exist, but it's actually a feature of git log:

git log -p [-m] [--follow] [-1] <path>

Note that -p can also be used to show the inline diff from a single commit:

git log -p -1 <commit>

Options used:

• -p (also -u or --patch) is hidden deeeeeeeep in the git-log man page, and is actually a display option for git-diff. When used with log, it shows the patch that would be generated for each commit, along with the commit information—and hides commits that do not touch the specified <path>. (This behavior is described in the paragraph on --full-diff, which causes the full diff of each commit to be shown.)
• -m causes merge commits to include the diff content (otherwise these just show the commit message, as if -p were not specified).
• -1 shows just the most recent change to the specified file (-n 1 can be used instead of -1); otherwise, all non-zero diffs of that file are shown.
• --follow is required to see changes that occurred prior to a rename.

As far as I can tell, this is the only way to immediately see the last set of changes made to a file without using git log (or similar) to either count the number of intervening revisions or determine the hash of the commit.

To see older revisions changes, just scroll through the log, or specify a commit or tag from which to start the log. (Of course, specifying a commit or tag returns you to the original problem of figuring out what the correct commit or tag is.)

Credit where credit is due:

• I discovered log -p thanks to this answer.
• Credit to FranciscoPuga and this answer for showing me the --follow option.
• Credit to ChrisBetti for mentioning the -n 1 option and atatko for mentioning the -1 variant.
• Credit to sweaver2112 for getting me to actually read the documentation and figure out what -p "means" semantically.
• Credit to Oscar Scholten for pointing out that by default, -p does not show diff-contents for merge commits.

One of the ways to use git diff is:

git diff <commit> <path>

And a common way to refer one commit of the last commit is as a relative path to the actual HEAD. You can reference previous commits as HEAD^ (in your example this will be 123abc) or HEAD^^ (456def in your example), etc ...

So the answer to your question is:

git diff HEAD^^ myfile