Add coverage for 'git format-patch --range-diff' single-patch case

In es/format-patch-rangediff, we added a '--range-diff' option to git-format-patch to automatically add a range-diff. We also added an option to write the diff as commentary to a single patch submission. However, this check was not tested.

I discovered this test gap by running 'make coverage-test coverage-report' on 'next' and then comparing the uncovered lines with those in the diff between 'master' and 'next'. I have a script that automates this process, and I'm still working on polishing it. You can see an earlier version at [1].

Based on es/format-patch-rangediff

[1]
https://github.com/derrickstolee/git/blob/coverage/contrib/coverage-diff.shA
bash script to report uncovered lines that were added in a diff.

Derrick Stolee (1):
t3206-range-diff.sh: cover single-patch case

 t/t3206-range-diff.sh | 5 +++++
 1 file changed, 5 insertions(+)

base-commit: 40ce41604daf200cdc85abded0133d40faafc2f8
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-37%2Fderrickstolee%2Frange-diff-test-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-37/derrickstolee/range-diff-test-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/37

Range-diff vs v1:

1: 58347a9624 ! 1: 277a4d2bd8 t3206-range-diff.sh: cover single-patch case

     @@ -22,7 +22,7 @@
       
      +test_expect_success 'format-patch --range-diff as commentary' '
      +	git format-patch --stdout --range-diff=HEAD~1 HEAD~1 >actual &&
     -+	grep -A 1 -e "\-\-\-" actual | grep "Range-diff:"
     ++	test_i18ngrep "^Range-diff:$" actual
      +'
      +
       test_done

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