tests: parse failures per command, non-ASCII paths, concurrent pushes !213

Control handlers were exercised end to end only.

• TestCommandParseFailures runs sixteen argv that every command must refuse before touching the store (unknown flag, missing value, too many arguments, a value out of range) and expects exit 2 with the usage in the message.
• TestNonASCIIPaths pushes dokumente/übersicht — ünïcode.txt and reads it through repo tree, cat, grep, blame and the web blob page. It found a defect: ls-tree returned the name as octal escapes, git's core.quotepath default. gitutil now sets core.quotepath=off through GIT_CONFIG_PARAMETERS for every git it runs, hooks included.
• TestConcurrentPushes lands six branches pushed at the same time and checks all six are there.

Stacked on !211; no code dependency.

Closes #129