Don't error out on git libraries at fixed tags

See #721 for context. This simply uses `git fetch` instead of `git pull`
if a specific version is requested. This is a kind of hacky solution,
since this won't work if somebody removes the `sync-version` from the
`fusesoc.conf`-file (in that case, the code will use `git pull` again,
which will fail as before). Since this usecase is not that common, this
commit should be enough for now.

fusesoc/provider/git.py

@@ -38,7 +38,8 @@ def init_library(library):
 
     @staticmethod
     def update_library(library):
-        git_args = ["-C", library.location, "pull"]
+        download_option = "pull" if not library.sync_version else "fetch"
+        git_args = ["-C", library.location, download_option]
         try:
             Git._checkout_library_version(library)
             Launcher("git", git_args).run()

tests/test_usecases.py

@@ -31,6 +31,30 @@ def test_git_library_with_default_branch_is_added_and_updated(caplog):
     assert "Updating..." in caplog.text
 
 
+def test_update_git_library_with_fixed_version(caplog, capsys):
+    """
+    Previously, one could not successfully use `fusesoc library update` on
+    libraries with specific sync versions. This test checks, that no error is
+    reported in that case.
+    """
+    url = "https://github.com/fusesoc/fusesoc-generators"
+    _fusesoc("library", "add", url, "--sync-version", "v0.1.4")
+    assert "Checked out fusesoc-generators at version v0.1.4" in caplog.text
+
+    _fusesoc("library", "list")
+    output = capsys.readouterr().out
+    assert "fusesoc-generators" in output
+    assert "v0.1.4" in output
+
+    _fusesoc("library", "update")
+    assert "Failed to update library" not in caplog.text
+
+    _fusesoc("library", "list")
+    output = capsys.readouterr().out
+    assert "fusesoc-generators" in output
+    assert "v0.1.4" in output
+
+
 def test_usage_error_leads_to_nonzero_exit_code():
     with pytest.raises(SystemExit):
         _fusesoc("--option_does_not_exist")