bug#8756: vc-git.el doesn't use --follow argument in vc-git-print-log

[Dmitry Dzhus]

How are you gentlemen,

Following renames is nice no matter what Git upstream thinks,
so I gave this bug a try. I'd like to share some progress.

Diffing (`d`), paging (`f`) and annotating a file (`a`) revision works.
I've tested it with Git version 1.8.5.4 and 2.0.2.

Parallel git log with hash/file information is run when
a log is queried. (vc-git-file-shalist) provides access to
a list of SHA1's/historical file names for the file open
in the *vc-change-log* buffer.

The bad part is that whole-changeset (`D`) diffing is broken
(wrong revisions are selected for diff).
The culprit is `vc-git-previous-revision`.
Using HASH^ to obtain the parent of HASH revision produces
unexpected results when file renames are involved. However,
for whole-changeset diffing HASH^ seems to be exactly what
we need. However, `vc-git-previous-revision` doesn't seem
to enable any inspection of the context in which we use it
(in other words, if we hit `d` or `D` in the log buffer.)
Any ideas?

I also have not tackled revision navigation in the annotation
mode, but this can likely be solved by plugging calls to 
`(vc-git-rev-to-filename)` somewhere in the annotation mode.

vc-git-follow-renames.patch
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