cmd/go/internal/modfetch/codehost: set LC_ALL=C for VCS commands

[JackDanger]

When the go command shells out to version control tools (git, hg, svn,
fossil, bzr), the error messages come back in whatever locale the user
has configured. This makes error message parsing unreliable for users
with non-English locales.

Set LC_ALL=C in the environment for all VCS subprocess invocations so
that error output is always in English and can be parsed consistently.

This resolves the TODO comment that was in the code requesting exactly
this change.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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