Bug 643397: Miapp uses a stale RepoBranchName

[AleksanderGladkov]

ISSUE:
Initialize NAV enlistment, cd App/BCApps, run Invoke-Miapp.
It fails with the error: Exception: fatal: HEAD does not point to a branch
Miapp fails in BCApps when RepoBranchName contains a stale value from another repository, such as master from NAV.

CAUSE:
Miapp trusts the environment value without checking whether the branch exists on the current repository's origin.

SOLUTION:
Preserve valid explicit base branches. When the configured branch does not exist on origin, show a warning and fall back to the current repository's origin/HEAD.

TESTS:
Validated valid, unset, and stale base-branch values. Verified that Invoke-Miapp automatically uses main when the inherited value is master.

Fixes AB#643397