PHP casts an array key which is a string that contains a decimal integer automatically into int type.
Quoting https://www.php.net/manual/en/language.types.array.php#language.types.array.syntax: "Strings containing valid decimal ints, unless the number is preceded by a + sign, will be cast to the int type."

As we check git branch names via phutil_nonempty_string($key), this throws an exception.
Thus explicitly cast the int to string.

Closes T15640.

• Have a git repository with a branch whose name consists of digits only and does not start with 0.
• Go to a commit which belongs both to the main/master branch and such a numeric branch via http://phorge.localhost/diffusion/ABCD/branches/main/;1234567890abcdef1234567890abcdef12345678
• Additionally, visit the commit and see that the "Branches" does not load as it also belongs to a numeric branch.