Fix reading of package dates using git
The function now uses the first time a file 'opam' appeared in a directory 'package.version'.
package modifications within a released version won't be listed thus.
Merge pull request #77 from AltGr/fix-order-attempt

Fix reading of package dates using git
Merge pull request #76 from dsheets/master

Very good, thanks !
Fixed build for latest OPAM
Merge pull request #80 from AltGr/fix-build

Fixed build for latest OPAM