GPG: use key fingerprint suffix to compare id for signing key

GPG: use key fingerprint suffix to compare id for signing key

Check whether the value of the git config user.signingKey is a suffix
of the full fingerprint of the key. This was already used for finding
keys in secring.gpg, but not in pubring.kbx. This mechanism allows a
user to use any unique suffix to identify keys; to avoid needless
collisions it's recommended to use at least the last 16 characters of
the hex representation of the fingerprint, which is the key id.[1]

[1] https://tools.ietf.org/html/rfc4880#section-12.2

Bug: 545673
Change-Id: If6fb4879502b6ee4b8c26c21b2714aeac4e4670c
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>