Support git-crypt (#168)

Support is inherently provided by `enter`, which now supports a command.
I've added a `git-crypt` command, which is really just an alias
under-the-hood for "enter git-crypt".

test/conftest.py

@@ -89,6 +89,7 @@ def supported_commands():
         'decrypt',
         'encrypt',
         'enter',
+        'git-crypt',
         'gitconfig',
         'help',
         'init',

test/test_git_crypt.py

@@ -0,0 +1,42 @@
+"""Test git-crypt"""
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    'crypt',
+    [False, 'installed', 'installed-but-failed'],
+    ids=['not-installed', 'installed', 'installed-but-failed']
+)
+def test_git_crypt(runner, yadm, paths, tmpdir, crypt):
+    """git-crypt tests"""
+
+    paths.repo.ensure(dir=True)
+    bindir = tmpdir.mkdir('bin')
+    pgm = bindir.join('test-git-crypt')
+
+    if crypt:
+        pgm.write(f'#!/bin/sh\necho git-crypt ran\n')
+        pgm.chmod(0o775)
+    if crypt == 'installed-but-failed':
+        pgm.write('false\n', mode='a')
+
+    script = f"""
+        YADM_TEST=1 source {yadm}
+        YADM_REPO={paths.repo}
+        GIT_CRYPT_PROGRAM="{pgm}"
+        git_crypt "param1"
+    """
+
+    run = runner(command=['bash'], inp=script)
+
+    if crypt:
+        if crypt == 'installed-but-failed':
+            assert run.failure
+        else:
+            assert run.success
+        assert run.out.strip() == 'git-crypt ran'
+    else:
+        assert run.failure
+        assert f"command '{pgm}' cannot be located" in run.out
+    assert run.err == ''