build: handle exceptional case when litex/migen not deployed as git repo

litex/build/tools.py

@@ -95,7 +95,10 @@ def get_migen_git_revision():
     import migen
     d = os.getcwd()
     os.chdir(os.path.dirname(migen.__file__))
+    try:
         r = subprocess.check_output(["git", "rev-parse", "--short", "HEAD"])[:-1].decode("utf-8")
+    except:
+        r = "--------"
     os.chdir(d)
     return r
 
@@ -103,7 +106,10 @@ def get_litex_git_revision():
     import litex
     d = os.getcwd()
     os.chdir(os.path.dirname(litex.__file__))
+    try:
         r = subprocess.check_output(["git", "rev-parse", "--short", "HEAD"])[:-1].decode("utf-8")
+    except:
+        r = "--------"
     os.chdir(d)
     return r