Added action to prevent changing msgid (#1351)

See discussion in #1344.

This checks that all `.po` files changed in pull requests either contain
changes in POT-Creation-Date (i.e. are run by the specific tool which
should change msgid's), or don't change msgid's (by checking that each
line changed is NOT between msgid and msgstr)

I have tested this on my fork, see [valid
changes](https://github.com/noamzaks/comprehensive-rust/pull/1/files)
where the action passes, and [invalid
changes](https://github.com/noamzaks/comprehensive-rust/pull/2/files)
where it doesn't.

The action also has output for failing/passing:

[passing](https://github.com/noamzaks/comprehensive-rust/actions/runs/6488171290/job/17620037473?pr=1)

[failing](https://github.com/noamzaks/comprehensive-rust/actions/runs/6488175171/job/17620050716?pr=2)

.github/workflows/prevent-msgid.py

@@ -0,0 +1,36 @@
+import os
+
+for filename in os.popen("git diff --name-only").read().split():
+    if not filename.endswith(".po"):
+        continue
+
+    if "POT-Creation-Date" in os.popen(f"git diff --unified=0 {filename}").read():
+        print(f"Assuming {filename} was changed automatically, skipping msgid check")
+        continue
+
+    changed_lines = {
+        i + 1
+        for i, line in enumerate(os.popen(f"git blame {filename}").readlines())
+        if line.startswith("00000000")
+    }
+
+    saw_msgid = False
+    with open(filename, "r") as f:
+        line = f.readline()
+        line_number = 1
+
+        while line:
+            if line.startswith("msgid"):
+                saw_msgid = True
+            elif line.startswith("msgstr"):
+                saw_msgid = False
+
+            if saw_msgid and line_number in changed_lines:
+                print(f"Changed msgid in file {filename}:{line_number}!")
+                print(
+                    "Please read https://github.com/google/comprehensive-rust/blob/main/TRANSLATIONS.md#creating-and-updating-translations."
+                )
+                exit(1)
+
+            line_number += 1
+            line = f.readline()

.github/workflows/prevent-msgid.yml

@@ -0,0 +1,21 @@
+name: Prevent msgid changes in translations
+
+on:
+  pull_request:
+    paths:
+      - "**/*.po"
+
+jobs:
+  prevent-msgid-changes:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Reset git
+        run: git reset origin/main
+
+      - name: Check po file changes
+        run: python3 .github/workflows/prevent-msgid.py