ci: run expensive tests on push builds to integration branches

Derrick Stolee suggested [1] that expensive tests should be run at a
regular cadence rather than on every PR iteration. Gate GIT_TEST_LONG
on push builds to the integration branches (next, master, main, maint)
so that the EXPENSIVE prereq is satisfied there but not during PR
validation, where the extra minutes of wall-clock time do not justify
themselves.

[1] https://lore.kernel.org/git/e1e8837f-7374-4079-ba87-ab95dd156e33@gmail.com/

Helped-by: Derrick Stolee <derrickstolee@github.com>
Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dscho

ci/lib.sh

@@ -314,6 +314,15 @@ export DEFAULT_TEST_TARGET=prove
 export GIT_TEST_CLONE_2GB=true
 export SKIP_DASHED_BUILT_INS=YesPlease
 
+# Enable expensive tests on push builds to integration branches, but
+# not on PR builds where the extra time is not justified for every
+# iteration.
+case "$GITHUB_EVENT_NAME,$CI_BRANCH" in
+push,*next*|push,*master*|push,*main*|push,*maint*)
+	export GIT_TEST_LONG=YesPlease
+	;;
+esac
+
 case "$distro" in
 ubuntu-*)
 	# Python 2 is end of life, and Ubuntu 23.04 and newer don't actually