sshkeeper v0.7.5 — repair partially migrated SSH key paths

v0.7.5 fixes one remaining cross-platform upgrade state that can occur after
syncing on v0.7.1-v0.7.4.

Fixed

Repair a legacy profile when its key record is already portable. A sync
bundle can legitimately end up half-migrated: the key is stored as
~/.ssh/id_rsa, while an unchanged server profile still contains a path
such as /home/user/.ssh/id_rsa. Windows now recognizes that both names
refer to the same synchronized key, writes the key under the local
%USERPROFILE%\.ssh directory, and rewrites IdentityFile accordingly.
Regression coverage for the real upgrade state. The test suite now
models an already-synced Windows client whose key record has migrated but
whose server record has not.

No database migration or re-pairing is required. Run synchronization once
after upgrading.