This changes git_signature_dup to actually honor oom conditions raised by
the call to git__strdup. It also aligns it with the error code return
pattern used everywhere else.

This was a convenience method for the refs front-end to do the reflog
writing. This is now done in the backend and it has no more reason for
being.

These functions act purely on the reflog data structure.

Provide a function that reads a reflog, performs an append and writes back to the
backend in one call.

References and their logs are logically coupled, let's make it so in
the code by moving the fs-based reflog implementation to live next to
the fs-based refs one.

As part of the change, make the function take names rather than
references, as only the names are relevant when looking up and
handling reflogs.

Implicit type conversion argument of function to size_t type
Suspicious sequence of types castings: size_t -> int -> size_t
Consider reviewing the expression of the 'A = B == C' kind. The expression is calculated as following: 'A = (B == C)'
Unsigned type is never < 0