This introduces a config option which enabled stores the requirements on a
repository in store instead.
When enabled, .hg/requires will contain the 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
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.