The work done on the new patcher was meant to avoid the case where

strings were used instead of integers for path prefix levels. However,

the work here was incomplete.
SCMClient.apply_patch() does normalize strings to integers, which is

correct when applying patches using that method. For a brief period of

time, Subversion (which was susceptible to the string prefix level bug)

benefited from this, but when it was converted to Patcher, this

workaround was lost. It's worth noting that apply_patch(), for

historical reasons, does expect a string, but not all SCMs handled

this right.
A core issue here was that rbt patch --px was configured to save the

value as a string, not an integer. Still, though, given

apply_patch()'s expectations, this wasn't technically incorrect until

now. With the new Patcher API, we do now set this to store as an

integer explicitly, and assert this.
Patch() has been updated to decode string-wrapped integers, with a

deprecation warning. This ensures that any other calls that result in a

Patch object will normalize to the right type.

Unit tests pass.
Manually tested rbt patch --px with integers, invalid values, and with

the argument omitted.