The repository form used to fully wipe the Repository.extra_data

field, which meant that any custom data would be gone, including any set

within the Internal State section of the form. That really meant that,

unlike most other objects, this extra_data was useless for storing

custom information.
This change improves this greatly by preserving any custom content in

this field. We no longer wipe the field completely, but rather only wipe

keys that correspond to the SCMTool or hosting service previously used

for the repository, or information relevant to the bug tracker, leaving

all else behind.
It is important to note that keys that start with bug_tracker,

SCMTool, or hosting service prefix are all likely to be removed when

saving the form, so custom keys should not use those prefixes.

Unit tests pass.
Saving several repositories, changing hosting services and SCMTools

and custom keys. Verified the stored extra_data appeared correct

in all cases.