repoview: remove unreachable code Found using Cython.

pvec: use the correct name for an identifier Found using Cython.

repoview: remove unreachable code Found using Cython.

mail: add missing import of sys Found using Cython.

statichttprepo: add missing import of os Found using Cython.

worker: add missing import of errno Found using Cython.

histedit: support editing of the first commit (issue3767)

test-commit-amend.t: fix check-code violation from 3a72c89a83ec

util: improve doc for checkcase

scheme: don't crash on invalid URLs

amend: support amending merge changesets (issue3778)

worker: catch all exceptions, try to exit usefully/safely

lock: if we fork, ensure that only the parent releases This prevents us from having a bunch of errant worker processes all try to release a lock if a problem occurs. (Releasing the lock more than once is harmless; it's invoking the associated callbacks we want to avoid.)

tag: clarify cryptic error message when tagging null revision

setup: make error message for missing Python headers more helpful