Avoid parsing index files more than once

In huge repositories, readIdxFile() can take quite a long time. Since
all index files are already read by OpenRepository(), it's useless to
re-parse them when they are needed: we can just re-use what was read the
first time.
Merge pull request #4 from Schnouki/master

Avoid parsing index files more than once
Mirror fix
Merge branch 'master' of github.com:gogits/git into dev