jenkins: always resolve tags to rev of the commit that got tagged

FC-41962
Closes #200

Previously, we always resolved tags to the commit rev. However if this
failed, there was a fallback to the rev of the tag (the output of `git
ls-remote` without the `^{}` modifier).

This is a problem in a deployment where we rely on the rev being the
commit rev since that's part of the S3 url where the frontend assets are
stored. The reason for the fallback that lead to such a deployment
error was a timing issue where the first `ls-remote` failed like this

    ssh: Could not resolve hostname gitlab.customer.com: Try again
    fatal: Could not read from remote repository.
    Please make sure you have the correct access rights
    and the repository exists.

and the second one (resolving to the tag's rev) succeeded.

We now call `ls-remote` without the `^{}` modifier and check if the
refspec got resolved to a tag (i.e. `refs/tags/...`). If that's the
case, a second `ls-remote` is issued with the `^{}` modifier to
get the rev of the commit that got tagged. If this second call fails,
we fail hard to make sure we don't get incorrect results silently.

CHANGES.d/20241121_122503_mb_FC_41962_tag_resolution.md

@@ -0,0 +1,3 @@
+- Correctness fix for `jenkins set-version`: if a tag is resolved, make sure it's _always_
+  resolved to the rev of the tagged commit (instead of the tag's rev) or fail hard to avoid
+  incorrect revs.

src/batou_ext/jenkins.py

@@ -7,6 +7,15 @@
 import sys
 
 
+def git_ls_remote(url, ref):
+    cmd = subprocess.Popen(
+        ["git", "ls-remote", url, ref], stdout=subprocess.PIPE
+    )
+    stdout, _ = cmd.communicate()
+    if stdout:
+        return stdout.decode("ascii").split("\t", 1)
+
+
 def git_resolve(url, version):
     if len(version) == 40:
         # revision.
@@ -16,19 +25,21 @@ def git_resolve(url, version):
             pass
         else:
             return version
-    # Symbolic name?
-    cmd = subprocess.Popen(
-        ["git", "ls-remote", url, version + "^{}"], stdout=subprocess.PIPE
-    )
-    stdout, stderr = cmd.communicate()
-    # if its not a tag, start another more generic attempt
-    if not stdout:
-        cmd = subprocess.Popen(
-            ["git", "ls-remote", url, version], stdout=subprocess.PIPE
-        )
-        stdout, stderr = cmd.communicate()
-    stdout = stdout.decode("ascii")
-    return stdout.split("\t", 1)[0]
+
+    ls_remote = git_ls_remote(url, version)
+    if ls_remote:
+        rev, full_refspec = ls_remote
+        # If full_refspec indicates that a tag was found, retry with `^{}`
+        # to make sure we get the rev of the commit instead of the tag's rev.
+        if full_refspec.startswith("refs/tags"):
+            ls_remote_tag = git_ls_remote(url, f"{version}^{{}}")
+            if ls_remote_tag:
+                rev = ls_remote_tag[0]
+            else:
+                raise ValueError(
+                    f"git ls-remote {url} {version}^{{}} failed, see above for the error."
+                )
+        return rev
 
 
 class VersionsUpdater: