treewide: Update prepare-commit-msg hook to strip prog/lang-* prefix

This behavior was documented in script documentation, but wasn't implemented
before.

git-hooks/prepare-commit-msg

@@ -24,6 +24,7 @@ count the sub-project prefix.
 """
 
 import sys
+import re
 from os.path import commonpath, dirname, exists, join as joinpath
 from subprocess import check_output
 args = {ix: value for ix, value in enumerate(sys.argv)}
@@ -68,7 +69,8 @@ with open(message_file, "w") as output:
 
 
     if common:
-        output.write(f"{common}: \n\n")
+        prefix = re.sub("prog/lang-[a-z]+/", "", common)
+        output.write(f"{prefix}: \n\n")
     else:
         output.write("treewide: \n\n")