perf(hg_branch): Replace direct calls to `hg` (#803)

perf: don't use hg process to get branch / bookmark information, each call was taking 250ms and it was making on average 2 calls.
the branch and bookmark information is available in files in the root of the repo, use that instead.
now takes 300 micro seconds

fix: do not use .hgignore to decide if it is an hg repo. was providing false positives. #721

fix: after changing branch the new branch was not show (the old branch was shown), the new branch is now correctly shown. #722

fix: if no branch override was set it would say (no branch) however if you commited in that state you would commit to the branch default so say that instead

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