We should be passing GIT_EUSER to libgit2 and check for GIT_EUSER when the function returns.
Stash.xs: Ignore GIT_EUSER, its not an error
* Tag and Config may have been affected similarly. If the user
returns a non-zero value in the callback, pass GIT_EUSER to
libgit2 regardless of the actual value returned by the user.
Coverage decreased (-0.09%) when pulling 2a13e98 on jacquesg/user-callback into e5bb418 on master.