t7606 (merge-theirs): modernize style

Guard setup commands with test_expect_success, so they are easier
to visually skip over and get to the good part.  While at it:

 - use test_commit for brevity and reproducible object names;

 - use test_cmp instead of using the test builtin to compare the
   result of command substitution, for better output with -v on
   failure.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7606-merge-custom.sh

@@ -1,46 +1,60 @@
 #!/bin/sh
 
-test_description='git merge
+test_description="git merge
 
-Testing a custom strategy.'
+Testing a custom strategy.
+
+*   (HEAD, master) Merge commit 'c2'
+|\
+| * (tag: c2) c2
+* | (tag: c1) c1
+|/
+* (tag: c0) c0
+"
 
 . ./test-lib.sh
 
-cat >git-merge-theirs <<EOF
-#!$SHELL_PATH
-eval git read-tree --reset -u \\\$\$#
-EOF
-chmod +x git-merge-theirs
-PATH=.:$PATH
-export PATH
+test_expect_success 'set up custom strategy' '
+	cat >git-merge-theirs <<-EOF &&
+	#!$SHELL_PATH
+	eval git read-tree --reset -u \\\$\$#
+	EOF
+
+	chmod +x git-merge-theirs &&
+	PATH=.:$PATH &&
+	export PATH
+'
 
 test_expect_success 'setup' '
-	echo c0 >c0.c &&
-	git add c0.c &&
-	git commit -m c0 &&
-	git tag c0 &&
-	echo c1 >c1.c &&
-	git add c1.c &&
-	git commit -m c1 &&
-	git tag c1 &&
-	git reset --hard c0 &&
+	test_commit c0 c0.c &&
+	test_commit c1 c1.c &&
+	git reset --keep c0 &&
 	echo c1c1 >c1.c &&
-	echo c2 >c2.c &&
-	git add c1.c c2.c &&
-	git commit -m c2 &&
-	git tag c2
+	git add c1.c &&
+	test_commit c2 c2.c
 '
 
 test_expect_success 'merge c2 with a custom strategy' '
 	git reset --hard c1 &&
+
+	git rev-parse c1 >head.old &&
+	git rev-parse c2 >second-parent.expected &&
+	git rev-parse c2^{tree} >tree.expected &&
 	git merge -s theirs c2 &&
-	test "$(git rev-parse c1)" != "$(git rev-parse HEAD)" &&
-	test "$(git rev-parse c1)" = "$(git rev-parse HEAD^1)" &&
-	test "$(git rev-parse c2)" = "$(git rev-parse HEAD^2)" &&
-	test "$(git rev-parse c2^{tree})" = "$(git rev-parse HEAD^{tree})" &&
+
+	git rev-parse HEAD >head &&
+	git rev-parse HEAD^1 >first-parent &&
+	git rev-parse HEAD^2 >second-parent &&
+	git rev-parse HEAD^{tree} >tree &&
+	git update-index --refresh &&
 	git diff --exit-code &&
 	git diff --exit-code c2 HEAD &&
 	git diff --exit-code c2 &&
+
+	! test_cmp head.old head &&
+	test_cmp head.old first-parent &&
+	test_cmp second-parent.expected second-parent &&
+	test_cmp tree.expected tree &&
 	test -f c0.c &&
 	grep c1c1 c1.c &&
 	test -f c2.c