diff.groovy: improve clone time by depth=1

[romani]

It is not big deal for CIs, it was a problem for me on hotspot, clone did not finish even for 10 minutes.... so I was not able to reuse my free time on the go to assist with PR problem in main repo.

✔ ~/java/github/checkstyle/contribution/checkstyle-tester/repositories [i816-status L|✚ 1…2⚑ 1] 
17:23 $ time git clone [email protected]:google/guava.git
Клонирование в «guava»…
remote: Enumerating objects: 398278, done.
remote: Counting objects: 100% (319/319), done.
remote: Compressing objects: 100% (127/127), done.
remote: Total 398278 (delta 172), reused 273 (delta 137), pack-reused 397959
Получение объектов: 100% (398278/398278), 436.55 MiB | 3.28 MiB/s, готово.
Определение изменений: 100% (295113/295113), готово.
Проверка соединения… готово.

real	2m40.251s
user	1m1.128s
sys	0m5.592s
✔ ~/java/github/checkstyle/contribution/checkstyle-tester/repositories [i816-status L|✚ 1…2⚑ 1] 
17:26 $ 
✔ ~/java/github/checkstyle/contribution/checkstyle-tester/repositories [i816-status L|✚ 1…2⚑ 1] 
17:26 $ rm -rf guava/
✔ ~/java/github/checkstyle/contribution/checkstyle-tester/repositories [i816-status L|✚ 1…2⚑ 1] 
17:26 $ #time git clone --depth 1 --branch v28.2 [email protected]:google/guava.git
✔ ~/java/github/checkstyle/contribution/checkstyle-tester/repositories [i816-status L|✚ 1…2⚑ 1] 
17:27 $ time git clone --depth 1 --branch v28.2 [email protected]:google/guava.git
Клонирование в «guava»…
remote: Enumerating objects: 2572, done.
remote: Counting objects: 100% (2572/2572), done.
remote: Compressing objects: 100% (1513/1513), done.
remote: Total 2572 (delta 1321), reused 1586 (delta 889), pack-reused 0
Получение объектов: 100% (2572/2572), 3.53 MiB | 3.06 MiB/s, готово.
Определение изменений: 100% (1321/1321), готово.
Проверка соединения… готово.
Note: checking out 'a1b3c06876803a0b0e5d2f16708e1328da1bac09'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>


real	0m2.594s
user	0m0.752s
sys	0m0.184s

Example from web to explain some items: https://stackoverflow.com/a/39067940/1015848

[rnveach]

Duplicate of https://github.com/checkstyle/contribution/issues/351 with same concerns