t/t5510-fetch.sh: improve testing with explicit URL and merge spec

Commit 6106ce46 introduced a test to demonstrate fetch's failure to
retrieve any objects or update FETCH_HEAD when it was supplied a repository
URL and the current branch had a configured merge spec.  This commit
expands the original test based on comments from Junio Hamano.  In addition
to actually verifying that the fetch updates FETCH_HEAD correctly, and does
not update the current branch, two more tests are added to ensure that the
merge configuration is ignored even when the supplied URL matches the URL
of the remote configured for the branch.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -240,10 +240,36 @@ test_expect_success 'fetch with a non-applying branch.<name>.merge' '
 	git fetch blub
 '
 
-test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge' '
+# URL supplied to fetch does not match the url of the configured branch's remote
+test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge [1]' '
+	one_head=$(cd one && git rev-parse HEAD) &&
+	this_head=$(git rev-parse HEAD) &&
 	git update-ref -d FETCH_HEAD &&
 	git fetch one &&
-	git rev-parse --verify FETCH_HEAD
+	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
+	test $this_head = "$(git rev-parse --verify HEAD)"
+'
+
+# URL supplied to fetch matches the url of the configured branch's remote and
+# the merge spec matches the branch the remote HEAD points to
+test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge [2]' '
+	one_ref=$(cd one && git symbolic-ref HEAD) &&
+	git config branch.master.remote blub &&
+	git config branch.master.merge "$one_ref" &&
+	git update-ref -d FETCH_HEAD &&
+	git fetch one &&
+	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
+	test $this_head = "$(git rev-parse --verify HEAD)"
+'
+
+# URL supplied to fetch matches the url of the configured branch's remote, but
+# the merge spec does not match the branch the remote HEAD points to
+test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge [3]' '
+	git config branch.master.merge "${one_ref}_not" &&
+	git update-ref -d FETCH_HEAD &&
+	git fetch one &&
+	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
+	test $this_head = "$(git rev-parse --verify HEAD)"
 '
 
 # the strange name is: a\!'b