fix usage without vcs repo

src/pycalver/vcs.py

@@ -34,6 +34,7 @@ VCS_SUBCOMMANDS_BY_NAME = {
         'commit'      : "git commit --file {path}",
         'tag'         : "git tag --annotate {tag} --message {tag}",
         'push_tag'    : "git push origin {tag}",
+        'show_remotes': "git config --get remote.origin.url",
     },
     'hg': {
         'is_usable'   : "hg root",
@@ -44,6 +45,7 @@ VCS_SUBCOMMANDS_BY_NAME = {
         'commit'      : "hg commit --logfile",
         'tag'         : "hg tag {tag} --message {tag}",
         'push_tag'    : "hg push {tag}",
+        'show_remotes': "hg paths",
     },
 }
 
@@ -71,6 +73,9 @@ class VCS:
     @property
     def is_usable(self) -> bool:
         """Detect availability of subcommand."""
+        if not os.path.exists(f".{self.name}"):
+            return False
+
         cmd = self.subcommands['is_usable'].split()
 
         try:
@@ -82,8 +87,19 @@ class VCS:
                 return False
             raise
 
+    @property
+    def has_remote(self) -> bool:
+        try:
+            output = self('show_remotes')
+            if output.strip() == "":
+                return False
+            return True
+        except Exception:
+            return False
+
     def fetch(self) -> None:
         """Fetch updates from remote origin."""
+        if self.has_remote:
             self('fetch')
 
     def status(self) -> typ.List[str]:
@@ -130,6 +146,7 @@ class VCS:
 
     def push(self, tag_name: str) -> None:
         """Push changes to origin."""
+        if self.has_remote:
             self('push_tag', tag=tag_name)
 
     def __repr__(self) -> str: