t3501: remove redundant file check and stop ignoring git <cmd> exit code

In the test 'cherry-pick after renaming branch', stop checking for
the presence of a file (opos) because we are going to "grep" in it in
the same test and the lack of it will be noticed as a failure anyway.

In the test 'revert after renaming branch', instead of allowing any
random contents as long as a known phrase is not there in it, we can
expect the exact outcome---after the successful revert of
"added", the contents of file "spoo" should become identical to
what was in file "oops" in the "initial" commit.

In both tests, we would not notice when "git rev-parse" starts
segfaulting without emitting any output.  The 'test' command would
end up being just "test =", which yields success. Therefore we could
use test_cmp_rev

Signed-off-by: Labnann <khalid.masum.92@gmail.com>

t/t3501-revert-cherry-pick.sh

@@ -66,8 +66,7 @@ test_expect_success 'cherry-pick after renaming branch' '
 
 	git checkout rename2 &&
 	git cherry-pick added &&
-	test $(git rev-parse HEAD^) = $(git rev-parse rename2) &&
-	test -f opos &&
+	test_cmp_rev rename2 HEAD^ &&
 	grep "Add extra line at the end" opos &&
 	git reflog -1 | grep cherry-pick
 
@@ -77,9 +76,8 @@ test_expect_success 'revert after renaming branch' '
 
 	git checkout rename1 &&
 	git revert added &&
-	test $(git rev-parse HEAD^) = $(git rev-parse rename1) &&
-	test -f spoo &&
-	! grep "Add extra line at the end" spoo &&
+	test_cmp_rev rename1 HEAD^ &&
+	test_cmp_rev initial:oops HEAD:spoo &&
 	git reflog -1 | grep revert
 
 '