mw-core-get: restore cwd after cd

The parsoidsvc-php-parsertests uses relative paths for MW_INSTALL_PATH.
Commit e6999b9 introduced an extra cd that changed the current working
directory causing subsequent cd to fail.

Use pushd to save the cwd and popd o restore it.

Example:

+ cd src/mediawiki/core
+ rm -rf '!(extensions)'
+ '[' -d /srv/ssd/gerrit/mediawiki/core.git ']'
+ git archive --remote=/srv/ssd/gerrit/mediawiki/core.git
+ tee /dev/fd/63
+ cd src/mediawiki/core
/srv/deployment/integration/slave-scripts/bin/mw-core-get.sh:
  line 39: cd: src/mediawiki/core: No such file or directory

Change-Id: Ice159991f061429980894f7da81772dbcacea8f9