CVS annotate.

The core GIT itself does not have a "cvs annotate" equivalent.
It has something that you may want to use when you would use
"cvs annotate".

Let's step back a bit and think about the reason why you would
want to do "cvs annotate a-file.c" to begin with.

You would use "cvs annotate" on a file when you have trouble
with a function (or even a single "if" statement in a function)
that happens to be defined in the file, which does not do what
you want it to do.  And you would want to find out why it was
written that way, because you are about to modify it to suit
your needs, and at the same time you do not want to break its
current callers.  For that, you are trying to find out why the
original author did things that way in the original context.

Many times, it may be enough to see the commit log messages of
commits that touch the file in question, possibly along with the
patches themselves, like this:

        $ git-whatchanged -p a-file.c

This will show log messages and patches for each commit that
touches a-file.

This, however, may not be very useful when this file has many
modifications that are not related to the piece of code you are
interested in.  You would see many log messages and patches that
do not have anything to do with the piece of code you are
interested in.  As an example, assuming that you have this piece
code that you are interested in in the HEAD version:

        if (frotz) {
                nitfol();
        }

you would use git-rev-list and git-diff-tree like this:

        $ git-rev-list HEAD |
          git-diff-tree --stdin -v -p -S'if (frotz) {
                nitfol();
        }'

We have already talked about the "--stdin" form of git-diff-tree
command that reads the list of commits and compares each commit
with its parents.  The git-whatchanged command internally runs
the equivalent of the above command, and can be used like this:

        $ git-whatchanged -p -S'if (frotz) {
                nitfol();
        }'

When the -S option is used, git-diff-tree command outputs
differences between two commits only if one tree has the
specified string in a file and the corresponding file in the
other tree does not.  The above example looks for a commit that
has the "if" statement in it in a file, but its parent commit
does not have it in the same shape in the corresponding file (or
the other way around, where the parent has it and the commit
does not), and the differences between them are shown, along
with the commit message (thanks to the -v flag).  It does not
show anything for commits that do not touch this "if" statement.

Also, in the original context, the same statement might have
appeared at first in a different file and later the file was
renamed to "a-file.c".  CVS annotate would not help you to go
back across such a rename, but GIT would still help you in such
a situation.  For that, you can give the -C flag to
git-diff-tree, like this:

        $ git-whatchanged -p -C -S'if (frotz) {
                nitfol();
        }'

When the -C flag is used, file renames and copies are followed.
So if the "if" statement in question happens to be in "a-file.c"
in the current HEAD commit, even if the file was originally
called "o-file.c" and then renamed in an earlier commit, or if
the file was created by copying an existing "o-file.c" in an
earlier commit, you will not lose track.  If the "if" statement
did not change across such rename or copy, then the commit that
does rename or copy would not show in the output, and if the
"if" statement was modified while the file was still called
"o-file.c", it would find the commit that changed the statement
when it was in "o-file.c".

[ BTW, the current versions of "git-diff-tree -C" is not eager
  enough to find copies, and it will miss the fact that a-file.c
  was created by copying o-file.c unless o-file.c was somehow
  changed in the same commit.]

You can use the --pickaxe-all flag in addition to the -S flag.
This causes the differences from all the files contained in
those two commits, not just the differences between the files
that contain this changed "if" statement:

        $ git-whatchanged -p -C -S'if (frotz) {
                nitfol();
        }' --pickaxe-all

[ Side note.  This option is called "--pickaxe-all" because -S
  option is internally called "pickaxe", a tool for software
  archaeologists.]