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

@@ -286,6 +286,8 @@ impl Error {
             GIT_EEOF,
             GIT_EINVALID,
             GIT_EUNCOMMITTED,
+            GIT_EDIRECTORY,
+            GIT_EMERGECONFLICT,
             GIT_PASSTHROUGH,
             GIT_ITEROVER,
             GIT_RETRY,
@@ -294,6 +296,9 @@ impl Error {
             GIT_EAPPLYFAIL,
             GIT_EOWNER,
             GIT_TIMEOUT,
+            GIT_EUNCHANGED,
+            GIT_ENOTSUPPORTED,
+            GIT_EREADONLY,
         )
     }