Add missing code checks to Error::raw_code()

These codes are both already handled by Error::code(), but because
they weren't listed in Error::raw_code(), it would return -1, and so
the paths in Error::code() for them would never be hit.  This fixes
the code of errors from Repository::merge_commits() with
fail_on_conflict set for me.

Author: alyssais

src/error.rs

@@ -289,6 +289,8 @@ impl Error {
             GIT_PASSTHROUGH,
             GIT_ITEROVER,
             GIT_RETRY,
+            GIT_EDIRECTORY,
+            GIT_EMERGECONFLICT,
             GIT_EMISMATCH,
             GIT_EINDEXDIRTY,
             GIT_EAPPLYFAIL,