[PATCH 7/7] setup_git_env: avoid blind fall-back to ".git"

When we default to ".git" without having done any kind of repository setup, the results quite often do what the user expects. But this has also historically been the cause of some poorly behaved corner cases. These cases can be hard to find, because they happen at the conjunction of two relatively rare circumstances:

  1. We are running some code which assumes there's a
     repository present, but there isn't necessarily one
     (e.g., low-level diff code triggered by "git diff
     --no-index" might try to look at some repository data).

  2. We have an unusual setup, like being in a subdirectory
     of the working tree, or we have a .git file (rather
     than a directory), or we are running a tool like "init"
     or "clone" which may operate on a repository in a
     different directory.

Our test scripts often cover (1), but miss doing (2) at the same time, and so the fallback appears to work but has lurking bugs. We can flush these bugs out by refusing to do the fallback entirely., This makes potential problems a lot more obvious by complaining even for "usual" setups.

This passes the test suite (after the adjustments in the previous patches), but there's a risk of regression for any cases where the fallback usually works fine but the code isn't exercised by the test suite. So by itself, this commit is a potential step backward, but lets us take two steps forward once we've identified and fixed any such instances.

Signed-off-by: Jeff King <peff@peff.net>
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 environment.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)