clean: make is_git_repository a public function

clean: make is_git_repository a public function

We have always had is_git_directory(), for looking at a
specific directory to see if it contains a git repo. In
0179ca7 (clean: improve performance when removing lots of
directories, 2015-06-15), we added is_git_repository() which
checks for a non-bare repository by looking at its ".git"
entry.

However, the fix in 0179ca7 needs to be applied other
places, too. Let's make this new helper globally available.
We need to give it a better name, though, to avoid confusion
with is_git_directory(). This patch does that, documents
both functions with a comment to reduce confusion, and
removes the clean-specific references in the comments.

Based-on-a-patch-by: Andreas Krey <a.krey@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>