This introduces a config option which enabled stores the requirements on a
repository in store instead.
When enabled, `.hg/requires` will contain the exp-share-safe requirement which
marks that the requirements are present in the store.
This is done so that repository requirements can be shared with shares made
using `hg share` command.
After this patch, `hg share` checks whether the source repository has
exp-share-safe requirement, if yes, it does not copy the requirements.
Test for the new functionality is added and a test case in exitsing share tests
is also added.