builtin/push: call set_refspecs after validating remote

When an end-user runs "git push" with an empty string for the remote
repository name, e.g.

    $ git push '' main

"git push" fails with a BUG(). Even though this is a nonsense request
that we want to fail, we shouldn't hit a BUG().  Instead we want to give
a sensible error message, e.g., 'bad repository'".

This is because since 9badf97c42 (remote: allow resetting url list,
2024-06-14), we reset the remote URL if the provided URL is empty. When
a user of 'remotes_remote_get' tries to fetch a remote with an empty
repo name, the function initializes the remote via 'make_remote'. But
the remote is still not a valid remote, since the URL is empty, so it
tries to add the URL alias using 'add_url_alias'. This in-turn will call
'add_url', but since the URL is empty we call 'strvec_clear' on the
`remote->url`. Back in 'remotes_remote_get', we again check if the
remote is valid, which fails, so we return 'NULL' for the 'struct
remote *' value.

The 'builtin/push.c' code, calls 'set_refspecs' before validating the
remote. This worked with empty repo names earlier since we would get a
remote, albeit with an empty URL. With the new changes, we get a 'NULL'
remote value, this causes the check for remote to fail and raises the
BUG in 'set_refspecs'.

Do a simple fix by doing remote validation first. Also add a test to
validate the bug fix. With this, we can also now directly pass remote to
'set_refspecs' instead of it trying to lazily obtain it.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5529-push-errors.sh

@@ -2,6 +2,9 @@
 
 test_description='detect some push errors early (before contacting remote)'
 
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
@@ -38,6 +41,20 @@ test_expect_success 'detect missing sha1 expressions early' '
 	test_cmp expect rp-ran
 '
 
+# We use an existing local_ref, since it follows a different flow in
+# 'builtin/push.c:set_refspecs()' and we want to test that regression.
+test_expect_success 'detect empty remote with existing local ref' '
+	test_must_fail git push "" main 2> stderr &&
+	grep "fatal: bad repository ${SQ}${SQ}" stderr
+'
+
+# While similar to the previous test, here we want to ensure that
+# even targeted refspecs are handled.
+test_expect_success 'detect empty remote with targeted refspec' '
+	test_must_fail git push "" HEAD:refs/heads/main 2> stderr &&
+	grep "fatal: bad repository ${SQ}${SQ}" stderr
+'
+
 test_expect_success 'detect ambiguous refs early' '
 	git branch foo &&
 	git tag foo &&