Add check that commit ID is valid hexadecimal

[Seb-MCaw]

At present,

alr publish https://host.invalid/path/to/repo.git 0000000000000000000000000000000000000000

merely complains about the untrusted host (as it should), but

alr publish https://host.invalid/path/to/repo.git zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

yields a rather unhelpful

error: Dynamic_Predicate failed at alire-origins.adb:435
error: alr encountered an unexpected error, re-run with -d for details.

The same issue arises with commits specified in the [origin] section of index manifests.

This PR adds an explicit check for non-hexadecimal characters alongside the existing check for an incorrect number of characters, so the new error message is

error: Could not complete the publishing assistant:
error:    invalid git commit id, 40 digits hexadecimal expected