[PATCH] meson: use test_environment conditionally.

[Sam James <sam@xxxxxxxxxx>]

test_environment is only defined with -Dtests, so use it conditionally
and define a stub environment() instead, to avoid erroring out:

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

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

Do the same for 'netrc' in contrib/ as it uses the same pattern.
---
Not sure if we can do better here somehow or not (with a disabler?)

 contrib/credential/netrc/meson.build | 8 ++++++--
 contrib/subtree/meson.build          | 8 ++++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/contrib/credential/netrc/meson.build b/contrib/credential/netrc/meson.build
index a990dbb86d..6d815d01c4 100644
--- a/contrib/credential/netrc/meson.build
+++ b/contrib/credential/netrc/meson.build
@@ -7,8 +7,12 @@ 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())
+else
+  credential_netrc_testenv = environment()
+fi
 
 test('t-git-credential-netrc',
   shell,
diff --git a/contrib/subtree/meson.build b/contrib/subtree/meson.build
index 9c72b23625..d18f188216 100644
--- a/contrib/subtree/meson.build
+++ b/contrib/subtree/meson.build
@@ -12,8 +12,12 @@ 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())
+else
+  subtree_test_environment = environment()
+endif
 
 test('t7900-subtree', shell,
   args: [ 't7900-subtree.sh' ],
-- 
2.48.1