The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.
In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.
The default merge_file level was XDL_MERGE_MINIMAL, which will
produce conflicts where there should not be in the case where
both sides were changed identically. Change the defaults to be
more aggressive (XDL_MERGE_ZEALOUS) which will more aggressively
compress non-conflicts. This matches git.git's defaults.
Increase testing around reverting a previously reverted commit to
illustrate this problem.
Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.
