Add testing with merge-ort merge strategy

In preparation for switching from merge-recursive to merge-ort as the
default strategy, have the testsuite default to running with merge-ort.
Keep coverage of the recursive backend by having the linux-gcc job run
with it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/run-build-and-tests.sh

@@ -16,6 +16,7 @@ linux-gcc)
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 	make test
 	export GIT_TEST_SPLIT_INDEX=yes
+	export GIT_TEST_MERGE_ALGORITHM=recursive
 	export GIT_TEST_FULL_IN_PACK_ARRAY=true
 	export GIT_TEST_OE_SIZE=10
 	export GIT_TEST_OE_DELTA_SIZE=5

t/test-lib.sh

@@ -448,6 +448,8 @@ export EDITOR
 
 GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-sha1}"
 export GIT_DEFAULT_HASH
+GIT_TEST_MERGE_ALGORITHM="${GIT_TEST_MERGE_ALGORITHM:-ort}"
+export GIT_TEST_MERGE_ALGORITHM
 
 # Tests using GIT_TRACE typically don't want <timestamp> <file>:<line> output
 GIT_TRACE_BARE=1