Retry transient Git config reads

[Tyrie Vella (tyrielv)]

Problem and Context

Concurrent Git processes can replace .git\config while VFS for Git reads it.
On Windows, a reader can receive a short Permission denied or sharing error during that replacement.
VFS for Git reads configuration through both git.exe and libgit2.
A retry in only one implementation would leave other config readers exposed to the same race.
No direct .git\config reader bypasses these two implementations.

Changes

• Add one bounded retry policy for all git.exe and libgit2 config reads.
• Retry only transient file-access failures, with five attempts and exponential delays from 20 to 160 milliseconds.
• Preserve missing-key results and fail non-transient errors without a retry.
• Keep config writes unchanged.
• Trace each retry with the config key, attempt number, retry limit, and delay.
• Buffer libgit2 multivalue results before callbacks, so a retry cannot duplicate callback effects.
• Add unit coverage for recovery, missing keys, persistent failures, non-transient failures, and libgit2 retries.