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

contrib/credential/netrc/meson.build

@@ -7,6 +7,7 @@ credential_netrc = custom_target(
   install_dir: get_option('libexecdir') / 'git-core',
 )
 
+if get_option('tests')
   credential_netrc_testenv = test_environment
   credential_netrc_testenv.set('CREDENTIAL_NETRC_PATH', credential_netrc.full_path())
 
@@ -18,3 +19,4 @@ test('t-git-credential-netrc',
     depends: test_dependencies + bin_wrappers + [credential_netrc],
     timeout: 0,
   )
+endif

contrib/subtree/meson.build

@@ -12,6 +12,7 @@ git_subtree = custom_target(
   install_dir: get_option('libexecdir') / 'git-core',
 )
 
+if get_option('tests')
   subtree_test_environment = test_environment
   subtree_test_environment.prepend('PATH', meson.current_build_dir())
 
@@ -22,6 +23,7 @@ test('t7900-subtree', shell,
     depends: test_dependencies + bin_wrappers + [ git_subtree ],
     timeout: 0,
   )
+endif
 
 if get_option('docs').contains('man')
   subtree_xml = custom_target(