pkg-auto: Fix syncing eclass files

eclass is a file, as opposed to the package, which is a directory. So
doing `git -C eclass/foo.eclass log -- .` will fail because we can't
do "cd eclass/foo.eclass", which is what `-C` is trying to do.

pkg_auto/impl/sync_with_gentoo.sh

@@ -169,7 +169,7 @@ function commit_with_gentoo_sha() {
     if [[ -z ${SKIP_GIT_INFO} ]]; then
         local commit
 
-        commit=$(git -C "${GENTOO}/${path}" log --pretty=oneline -1 -- . | cut -f1 -d' ')
+        commit=$(git -C "${GENTOO}" log --pretty=oneline -1 -- "${path}" | cut -f1 -d' ')
         commit_extra+=( --message "It's from Gentoo commit ${commit}." )
         unset commit
     fi