We usually check entries returned by `git_sortedcache_entry` for
NULL pointers. As we have a write lock in `packed_write`, though,
it really should not happen that the function returns NULL.

Assert that ref is not NULL to silence a Coverity warning.

When normalizing options we try to look up HEAD's OID. While this
action may fail in malformed repositories we never check the
return value of the function.

Fix the issue by converting `normalize_options` to actually
return an error and handle the error in `git_blame_file`.

Accessing the current values map is handled through the
`refcounder_strmap_take` function, which first acquires a mutex
before accessing its values. While this assures everybody is
trying to access the values with the mutex only we do not check
if the locking actually succeeds.

Fix the issue by checking if acquiring the lock succeeds and
returning `NULL` if we encounter an error. Adjust callers.

When writing to a file with locking not check if writing the
locked file actually succeeds. Fix the issue by returning error
code and message when writing fails.

Callers of `git_config__cvar` already handle the case where the
function returns an error due to a failed configuration variable
lookup, but we are actually swallowing errors when calling
`git_config__lookup_entry` inside of the function.

Fix this by returning early when `git_config__lookup_entry`
returns an error. As we call `git_config__lookup_entry` with
`no_errors == false` which leads us to call `get_entry` with
`GET_NO_MISSING` we will not return early when the lookup fails
due to a missing entry. Like this we are still able to set the
default value of the cvar and exit successfully.