Improved handling of git syntax names with no git repository

This makes "is_git_repository()" return non-NULL for all file names that
match the git name pattern, even if we don't find an actual git
repository there.  That way, we won't fall back to writing out an XML
file with an odd filename.

If there is no actual git repository, we return a special invalid dummy
pointer, and then the git reading and writing routines will catch it and
return the appropriate error.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Revert "If saving to a git repository fails, don't create oddly named XML file"

This reverts commit e70bbb637e1d

Linus' solution in commit 27c36ec4cf41 ("Improved handling of git syntax
names with no git repository") is much better and makes my hack
unnecessary.