[BUG] Move tracking in git diff is not as good as in git status

[Scott Schmit]

In my use of git, I've noticed that "git status" is a lot better at
tracking moves and renames than "git diff", and this has recently caused
me a lot of headaches because a large number of moves were made in a
single commit, and it was very difficult to figure out which moves were
right and which were wrong.

I was using a fairly old version of git (1.7.11), but was able to
reproduce it on git 2.2.1.

Here's a reproduction recipe:

#!/bin/sh -x

# Fill in your git.git working tree path here:
GIT_GIT_REPO=

if [ -z "$GIT_GIT_REPO" ]; then
    echo Fill in GIT_GIT_REPO
    exit 1
fi

git init mv-test
cd mv-test/

# Pick two sample files of non-trivial size, since files that are too small
# never get tracked as moves.
cp ${GIT_GIT_REPO}/Documentation/asciidoc.conf .
cp ${GIT_GIT_REPO}/Documentation/blame-options.txt .
git add *
git commit -m "Start with two files from git.git/Documentation"

# Now rename them to something. Using 1 & 2 because they're nice & short.
git mv asciidoc.conf 1
git mv blame-options.txt 2

# Status sees the rename
git status

# So does the summary on the commit
git commit -m "Rename both files"

# And move tracking works
git diff -M --stat --summary HEAD~..
git diff -C --stat --summary HEAD~..

# Now "shift" the files
git mv 2 3
git mv 1 2

# Status knows what's going on
git status

# So does commit
git commit -m "2=1;3=2;"

# Neither of these commands get it (but -C gets a glimmer of the truth)
git diff -M --stat --summary HEAD~..
git diff -C --stat --summary HEAD~..

# Swap the files in place
git mv 3 tmp
git mv 2 3
git mv tmp 2

# Status gets it
git status

# Commit understands
git commit -m "Swap 2 & 3"

# Diff has no idea
git diff -M --stat --summary HEAD~..
git diff -C --stat --summary HEAD~..

---

At first, I thought it was because the "git mv" command recorded
something in the index that's lost once the commit happens.

To check if that was so, I went back to the commit in question, did a
"git reset HEAD~1" and "git add -A ." and git status understood what was
going on just fine.

-- 
Scott Schmit

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