Blob hash of binary files in patches generated by git format patch show in full form instead of short form

[Bagas Sanjaya <bagasdotme@gmail.com>]

Thank you for filling out a Git bug report!
Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

I'm trying to do format-patch with binary files (images) in commits.
In each commit, it adds an image and its alt description text in separated
file.

Full steps:

   First, initialize empty repo and populate it with commits:
   - cd /tmp
   - mkdir bin-patch && cd bin-patch
   - git init
   - echo "test format-patch binary files" > README
   - git add * && git commit -m "init README"
   - git checkout -b test
   - wget -c [1] -O stackoverflow.png && echo "Stack Overflow" > stackoverflow.alt
   - git add * && git commit -m "Add Stack Overflow logo"
   - wget -c [2] -O idntm.jpg && echo "Indonesia's Next Top Model cast" > idntm.alt
   - git add * && git commit -m "Add IdNTM cast poster"

   Now prepare patches as usual (with cover letter ignored for this purpose):
   - git format-patch --cover-letter -M master

   (image link):
   [1]: https://upload.wikimedia.org/wikipedia/commons/thumb/0/02/Stack_Overflow_logo.svg/1280px-Stack_Overflow_logo.svg.png
   [2]: https://upload.wikimedia.org/wikipedia/en/9/9f/IndonesiaNTM1Cast.jpg

What did you expect to happen? (Expected behavior)

Blob hash in the `index` header of generated patches for binary file (image)
use short form (7 characters), just like for text file (alt description).

What happened instead? (Actual behavior)
Blob hash in the `index` stanza of generated patches for image use full
(long) form.

For first patch (Stack Overflow commit), diff header for the image read:
```
diff --git a/stackoverflow.png b/stackoverflow.png
new file mode 100644
index 0000000000000000000000000000000000000000..969908ad3161a66af31f2441cfea4ae002a5ec67
```

while diff header for alt description read:
```
diff --git a/stackoverflow.alt b/stackoverflow.alt
new file mode 100644
index 0000000..9368417
```

Similarly, for the second patch (INTM poster), diff header for the image read:
```
diff --git a/idntm.jpg b/idntm.jpg
new file mode 100644
index 0000000000000000000000000000000000000000..8921ab9540e0a36a53f8c6632482fb04d5d0cc6c
```

while diff header for alt description read:
```
diff --git a/idntm.alt b/idntm.alt
new file mode 100644
index 0000000..719feb9
```

What's different between what you expected and what actually happened?

Blob hash for binary files are shown in full form, as opposed to blob hash
for text files.

Anything else you want to add:
(none)

Please review the rest of the bug report below.
You can delete any lines you don't wish to share.


[System Info]
git version:
git version 2.31.0.29.g98164e9585
cpu: x86_64
built from commit: 98164e9585e02e31dcf1377a553efe076c15f8c6
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.11.6-kernelorg-upstream-generic #1 SMP Fri Mar 12 06:35:27 WIB 2021 x86_64
compiler info: gnuc: 9.3
libc info: glibc: 2.31
$SHELL (typically, interactive shell): /bin/bash


[Enabled Hooks]
(none)

-- 
An old man doll... just what I always wanted! - Clara