RFR: checkout bot: use full path for marks files

[Erik Helin]

Hi all,

please review this small fix that makes the checkout bot use the full path of the from repo as the name for the marks
file. I also had to update `HostedRepositoryStorage` to cope with files in directories.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - checkout bot: use full path for marks files

Changes: https://git.openjdk.java.net/skara/pull/745/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=745&range=00
  Stats: 4 lines in 3 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/745.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/745/head:pull/745

PR: https://git.openjdk.java.net/skara/pull/745