brew.sh: Use shims/scm/git for HOMEBREW_VERSION

[sjackman]

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Move the use of git to determine HOMEBREW_VERSION below the definition of the function git.

[alyssais]

Can you describe what the effects of this change are? I think I could guess but don't want to make a wrong assumption.

[MikeMcQuaid]

LGTM, thanks again @sjackman!

[sjackman]

Before this patch, the command

HOMEBREW_VERSION="$(git -C "$HOMEBREW_REPOSITORY" describe --tags --dirty --abbrev=7 2>/dev/null)"

is running using the git executable found in PATH.
After this patch, the command is run using the git shell function, which runs $HOMEBREW_LIBRARY/Homebrew/shims/scm/git.