bug#8756: 23.3; vc-git.el doesn't use --follow argument in vc-git-print-log

[Steinar Bang]

>>>>> Dan Nicolaescu <dann@gnu.org>:

> The simple fix applied was suggested with the bug report.  It is not
> enough to make it fully work, things like annotate, diff, etc do not
> work...

Indeed:
 http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8756#8
 http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8756#11

To summarize: with --follow in place, what's listed in the log is the
complete history, but annotate, and find files won't work past renames,
and diffs reports "no diffs" when there should have been one (past
renames).

To summarize my experiments further down in the bug report thread: if
you do  
 git diff -M sha1a sha1b -- path-of-file-in-sha1a path-of-file-in-sha1b
you get a result that is the diff of the two files.

Here sha1a is a sha1 in "git log --follow path-of-file" that refers to a
point in history before moving a file, and path-of-file-in-sha1a is the
path to the file at that time in history.  And sha1b and
path-of-file-in-sha1b refers to a point in time where the file has a
different name or location.

To get the sha1/path infomation one can do the command
 git log --follow --name-only --pretty=oneline 
path-to-current-version-of-the-file
and then parse out the sha1/path pairs.

Note: this "git log" has to be done in addition to the displayed "git
log".  This command is not for display, it is just to get the sha1/path
pairs. 

I have created an example repo that contains some renames, moves, and
plain changes, at: https://github.com/steinarb/renametest.git

Here is an example of a diff in that repository (in this case a file
that has been renamed once, and then renamed file has been moved to a
different location):

sb@edwards:~/git/renametest$ git diff -M 
e5984c6a9e8e015dc521fb6a1756a88d5ffd676d 
09d1bf6aa4dcd20dd36ca7668b75809e794428e0 -- hoved.txt application/main.txt
diff --git a/hoved.txt b/application/main.txt
similarity index 50%
rename from hoved.txt
rename to application/main.txt
index 9a57969..7f5e5a6 100644
--- a/hoved.txt
+++ b/application/main.txt
@@ -1,5 +1,6 @@
 Hello world!
-Version 1.0
-Feature1 implementation
+Version 2.0
 Feature2 implementation
 Feature3 implementation
+Feature4 implementation
+Feature4 implementation


Here is an example of the command to get the sha1/path pairs:
sb@edwards:~/git/renametest$ git log --follow --name-only --pretty=oneline 
application/main.txt
09d1bf6aa4dcd20dd36ca7668b75809e794428e0 Bumped the version number.
application/main.txt
313aa2f727fefc9e50354380cb9fb3a70401b5f3 Moved a single file to a different 
directory.
application/main.txt
9b47cdca956a59b4582c67d24f9474a576aff7b9 Changes to two files
main.txt
dd99483cda11b4ab040135767655c07c5cf07efb Removed a line.
main.txt
2b0f570422db1e0fac39697d908b31eb70e895f5 Renamed a file and changed the README 
in the same commit
main.txt
b35723cfbc91a3108469091c8c77ee91107e1ab8 Added a new feature.
hoved.txt
e5984c6a9e8e015dc521fb6a1756a88d5ffd676d Introduced a new file
hoved.txt