dvc: hooks only act in branches where dvc active (#2338)

* dvc: hooks only act in branches where dvc active

In a branch where DVC hasn't been initialized (as detected by
`git ls-files .dvc` not listing any files), don't invoke dvc in the git hooks.
This avoids trouble when switching between branches with and without dvc
initialized.

Fixes #2208

* dvc: simplify hooks per PR comment

dvc/scm/git/__init__.py

@@ -216,19 +216,17 @@ class Git(Base):
         return [t.name for t in self.git.tags]
 
     def _install_hook(self, name, cmd):
-        command = "dvc {}".format(cmd)
+        command = '[ -z "$(git ls-files .dvc)" ] || exec dvc {}'.format(cmd)
 
         hook = os.path.join(self.root_dir, self.GIT_DIR, "hooks", name)
 
         if os.path.isfile(hook):
             with open(hook, "r+") as fobj:
                 if command not in fobj.read():
-                    fobj.write("exec {command}\n".format(command=command))
+                    fobj.write("{command}\n".format(command=command))
         else:
             with open(hook, "w+") as fobj:
-                fobj.write(
-                    "#!/bin/sh\n" "exec {command}\n".format(command=command)
-                )
+                fobj.write("#!/bin/sh\n" "{command}\n".format(command=command))
 
         os.chmod(hook, 0o777)
 

tests/func/test_install.py

@@ -36,7 +36,11 @@ class TestInstall(object):
 
         assert main(["install"]) == 0
 
-        expected_script = "#!/bin/sh\n" "echo hello\n" "exec dvc checkout\n"
+        expected_script = (
+            "#!/bin/sh\n"
+            "echo hello\n"
+            '[ -z "$(git ls-files .dvc)" ] || exec dvc checkout\n'
+        )
 
         with open(self._hook("post-checkout"), "r") as fobj:
             assert fobj.read() == expected_script
@@ -45,7 +49,10 @@ class TestInstall(object):
         assert main(["install"]) == 0
         assert main(["install"]) == 0
 
-        expected_script = "#!/bin/sh\n" "exec dvc checkout\n"
+        expected_script = (
+            "#!/bin/sh\n"
+            '[ -z "$(git ls-files .dvc)" ] || exec dvc checkout\n'
+        )
 
         with open(self._hook("post-checkout"), "r") as fobj:
             assert fobj.read() == expected_script