Only call git on a git repository

Author: MarcusCalhoun-Lopez

tools/install.py

@@ -37,10 +37,11 @@
 
 
 def add_revision_file(target):
-  # text=True would be better than encoding here, but it's only supported in 3.7+
-  git_hash = subprocess.check_output(['git', 'rev-parse', 'HEAD'], encoding='utf-8').strip()
-  with open(os.path.join(target, 'emscripten-revision.txt'), 'w') as f:
-    f.write(git_hash + '\n')
+  if os.path.isdir('.git'):
+    # text=True would be better than encoding here, but it's only supported in 3.7+
+    git_hash = subprocess.check_output(['git', 'rev-parse', 'HEAD'], encoding='utf-8').strip()
+    with open(os.path.join(target, 'emscripten-revision.txt'), 'w') as f:
+      f.write(git_hash + '\n')
 
 
 def copy_emscripten(target):