git: suggest clearing cache on git log error

In some cases, the cache gets corrupted, showing:

pkgcheck scan: error: failed running git log: fatal: Invalid revision range <hash>..origin/HEAD

Recommend clearing the cache w/ `pkgcheck cache -R` as a workaround.
Signed-off-by: Martin Mokry <martin-kokos@protonmail.com>

src/pkgcheck/addons/git.py

@@ -126,7 +126,10 @@ def __next__(self):
         if not self._running:
             if self.proc.poll() or not line:
                 error = self.proc.stderr.read().decode().strip()
-                raise GitError(f"failed running git log: {error}")
+                if 'Invalid revision range' in error:
+                    raise GitError(f"failed running git log: {error}\nTry clearing the cache: pkgcheck cache -R")
+                else:
+                    raise GitError(f"failed running git log: {error}")
             self._running = True
             self.git_config.close()