meson: respect 'tests' build option in contrib

Both the "netrc" credential helper and git-subtree(1) from "contrib/"
carry a couple of tests with them. These tests get wired up in Meson
unconditionally even in the case where `-Dtests=false`. As those tests
depend on the `test_enviroment` variable, which only gets defined in
case `-Dtests=true`, the result is an error:

```
$ meson setup -Dtests=false -Dcontrib=subtree build
[...]

contrib/subtree/meson.build:15:27: ERROR: Unknown variable "test_environment".
```

Fix the issue by not defining these tests at all in case the "tests"
option is set to `false`.

Reported-by: Sam James <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

contrib/credential/netrc/meson.build

@@ -7,14 +7,16 @@ credential_netrc = custom_target(
   install_dir: get_option('libexecdir') / 'git-core',
 )
 
-credential_netrc_testenv = test_environment
-credential_netrc_testenv.set('CREDENTIAL_NETRC_PATH', credential_netrc.full_path())
+if get_option('tests')
+  credential_netrc_testenv = test_environment
+  credential_netrc_testenv.set('CREDENTIAL_NETRC_PATH', credential_netrc.full_path())
 
-test('t-git-credential-netrc',
-  shell,
-  args: [ meson.current_source_dir() / 't-git-credential-netrc.sh' ],
-  workdir: meson.current_source_dir(),
-  env: credential_netrc_testenv,
-  depends: test_dependencies + bin_wrappers + [credential_netrc],
-  timeout: 0,
-)
+  test('t-git-credential-netrc',
+    shell,
+    args: [ meson.current_source_dir() / 't-git-credential-netrc.sh' ],
+    workdir: meson.current_source_dir(),
+    env: credential_netrc_testenv,
+    depends: test_dependencies + bin_wrappers + [credential_netrc],
+    timeout: 0,
+  )
+endif

contrib/subtree/meson.build

@@ -12,16 +12,18 @@ git_subtree = custom_target(
   install_dir: get_option('libexecdir') / 'git-core',
 )
 
-subtree_test_environment = test_environment
-subtree_test_environment.prepend('PATH', meson.current_build_dir())
+if get_option('tests')
+  subtree_test_environment = test_environment
+  subtree_test_environment.prepend('PATH', meson.current_build_dir())
 
-test('t7900-subtree', shell,
-  args: [ 't7900-subtree.sh' ],
-  env: subtree_test_environment,
-  workdir: meson.current_source_dir() / 't',
-  depends: test_dependencies + bin_wrappers + [ git_subtree ],
-  timeout: 0,
-)
+  test('t7900-subtree', shell,
+    args: [ 't7900-subtree.sh' ],
+    env: subtree_test_environment,
+    workdir: meson.current_source_dir() / 't',
+    depends: test_dependencies + bin_wrappers + [ git_subtree ],
+    timeout: 0,
+  )
+endif
 
 if get_option('docs').contains('man')
   subtree_xml = custom_target(