Refactor internal routers (partial backport, auth token const time comparing) (#32473) (#32479)

Partially backport #32473. LFS related changes are not in 1.22, so skip
them.

1. Ignore non-existing repos during migrations
2. Improve ReadBatchLine's comment
3. Use `X-Gitea-Internal-Auth` header for internal API calls and make
the comparing constant time (it wasn't a serous problem because in a
real world it's nearly impossible to timing-attack the token, but indeed
security related and good to fix and backport)
4. Fix route mock nil check

modules/git/batch_reader.go

@@ -146,9 +146,8 @@ func catFileBatch(ctx context.Context, repoPath string) (WriteCloserError, *bufi
 }
 
 // ReadBatchLine reads the header line from cat-file --batch
-// We expect:
-// <sha> SP <type> SP <size> LF
-// sha is a hex encoded here
+// We expect: <oid> SP <type> SP <size> LF
+// then leaving the rest of the stream "<contents> LF" to be read
 func ReadBatchLine(rd *bufio.Reader) (sha []byte, typ string, size int64, err error) {
 	typ, err = rd.ReadString('\n')
 	if err != nil {