Fix difftool problem with intent-to-add files

This problem was reported in
https://github.com/git-for-windows/git/issues/2677, but the problem actually lies with git diff --raw, and it seems that the bug has been with us ever since --intent-to-add was introduced.

Changes since v4:

* Improved both commit messages after feedback from Junio.

Changes since v3:

* Instead of showing the same OID in git diff-files --raw as in git diff-files -p for intent-to-add files, we now imitate the logic for modified (non-intent-to-add) files: we show the "null" OID (i.e. all-zero).

* As a consequence, we no longer just undo the inadvertent change where intent-to-add files were reported with the empty tree instead of the empty blob, but we fix the bug that has been with us since run_diff_files() was taught about intent-to-add files, i.e. we fix the original bug of 425a28e0a4e (diff-lib: allow ita entries treated as "not yet exist in index", 2016-10-24), at long last.

Changes since v2:

* Now we also drop the no-longer-used definition of hash_t in t2203.

Changes since v1:

* Rebased onto sk/diff-files-show-i-t-a-as-new.
* Verified that sk/diff-files-show-i-t-a-as-new does not completely resolve the issue (the --raw output still claims the empty blob as the post-image, although the difftool symptom "went away").
* Amended the central patch of this PR to include a fix for the regression test that was introduced in sk/diff-files-show-i-t-a-as-new: it expected the raw diff to contain the hash of the empty tree object (which is incorrect no matter how you turn it: any hash in any raw diff should refer to blob objects).
* Reordered the patches so that the central patch comes first (otherwise, the "empty tree" fix would cause a test failure in t2203).

Johannes Schindelin (2):
diff-files --raw: show correct post-image of intent-to-add files difftool -d: ensure that intent-to-add files are handled correctly

 diff-lib.c            | 3 +--
 t/t2203-add-intent.sh | 5 ++---
 t/t7800-difftool.sh   | 8 ++++++++
 3 files changed, 11 insertions(+), 5 deletions(-)

base-commit: feea6946a5b746ff4ebf8ccdf959e303203a6011
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-654%2Fdscho%2Fdifftool-ita-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-654/dscho/difftool-ita-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/654

Range-diff vs v4:

1: 69256ab910 ! 1: 4392ae4be6 diff-files --raw: show correct post-image of intent-to-add files
@@ Commit message

[...] 0{40} if creation, unmerged or "look at work tree".

- This happens for example when showing modified, unstaged files.
+ on the right hand (i.e. postimage) side. This happens for files that
+ have unstaged modifications, and for files that are unmodified but
+ stat-dirty.

For intent-to-add files, we used to show the empty blob's hash instead. In c26022ea8f5 (diff: convert diff_addremove to struct object_id, 2017-05-30), we made that worse by inadvertently changing that to the hash of the empty tree.

- Let's make the behavior consistent with modified files by showing
+ Let's make the behavior consistent with files that have unstaged
+ modifications (which applies to intent-to-add files, too) by showing
all-zero values also for intent-to-add files.

Accordingly, this patch adjusts the expectations set by the regression
2: 9bb8d84ea9 ! 2: 5f933e852a difftool -d: ensure that intent-to-add files are handled correctly
@@ Commit message

In https://github.com/git-for-windows/git/issues/2677, a git difftool -d problem was reported. The underlying cause was a bug in `git
- diff-files --raw` that we just fixed.
+ diff-files --raw` that we just fixed: it reported intent-to-add files
+ with the empty _tree_ as the post-image OID, when we need to show
+ an all-zero (or, "null") OID instead, to indicate to the caller that
+ they have to look at the worktree file.
+
+ The symptom of that problem shown by git difftool was this:
+
+ error: unable to read sha1 file of <path> (<empty-tree-OID>)
+ error: could not write '<filename>'

Make sure that the reported difftool problem stays fixed.

gitgitgadget