ci: merge linux-gcc-default into linux-gcc

The "linux-gcc-default" job is mostly doing the same as the "linux-gcc"
job, except for a couple of minor differences:

  - We use an explicit GCC version instead of the default version
    provided by the distribution. We have other jobs that test with
    "gcc-8", making this distinction pointless.

  - We don't set up the Python version explicitly, and instead use the
    default Python version. Python 2 has been end-of-life for quite a
    while now though, making this distinction less interesting.

  - We set up the default branch name to be "main" in "linux-gcc". We
    have other testcases that don't and also some that explicitly use
    "master".

  - We use "ubuntu:20.04" in one job and "ubuntu:latest" in another. We
    already have a couple other jobs testing these respectively.

So overall, the job does not add much to our test coverage.

Drop the "linux-gcc-default" job and adapt "linux-gcc" to start using
the default GCC compiler, effectively merging those two jobs into one.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -271,7 +271,6 @@ jobs:
             pool: ubuntu-latest
           - jobname: linux-gcc
             cc: gcc
-            cc_package: gcc-8
             pool: ubuntu-20.04
           - jobname: linux-TEST-vars
             cc: gcc
@@ -286,9 +285,6 @@ jobs:
           - jobname: osx-gcc
             cc: gcc-13
             pool: macos-13
-          - jobname: linux-gcc-default
-            cc: gcc
-            pool: ubuntu-latest
           - jobname: linux-leaks
             cc: gcc
             pool: ubuntu-latest

.gitlab-ci.yml

@@ -46,14 +46,10 @@ test:linux:
       - jobname: linux-gcc
         image: ubuntu:20.04
         CC: gcc
-        CC_PACKAGE: gcc-8
       - jobname: linux-TEST-vars
         image: ubuntu:20.04
         CC: gcc
         CC_PACKAGE: gcc-8
-      - jobname: linux-gcc-default
-        image: ubuntu:latest
-        CC: gcc
       - jobname: linux-leaks
         image: ubuntu:latest
         CC: gcc

ci/lib.sh

@@ -328,11 +328,6 @@ export SKIP_DASHED_BUILT_INS=YesPlease
 
 case "$distro" in
 ubuntu-*)
-	if test "$jobname" = "linux-gcc-default"
-	then
-		break
-	fi
-
 	# Python 2 is end of life, and Ubuntu 23.04 and newer don't actually
 	# have it anymore. We thus only test with Python 2 on older LTS
 	# releases.