Remove Xgit.Util.FileSnapshot.

[scouten]

Changes in This Pull Request

This need is better served by Xgit.Util.ObservedFile.

This allows us to remove the dependency on ConCache.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes. n/a
• All cases where a literal value is returned use the cover macro to force code coverage. n/a
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a4963b4). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master   #286   +/-   ##
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  Coverage          ?   100%           
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  Files             ?     29           
  Lines             ?   1461           
  Branches          ?      0           
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  Hits              ?   1461           
  Misses            ?      0           
  Partials          ?      0

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