mailinfo: read local configuration

mailinfo: read local configuration

Since b9605bc4f2 ("config: only read .git/config from configured
repos", 2016-09-12), we do not read from ".git/config" unless we
know we are in a repository.  "git mailinfo" however didn't do the
repository discovery and instead relied on the old behaviour.  This
was mostly OK because it was merely run as a helper program by other
porcelain scripts that first chdir's up to the root of the working
tree.

Teach the command to run a "gentle" version of repository discovery
so that local configuration variables like mailinfo.scissors are
honoured.

Signed-off-by: Junio C Hamano <gitster@pobox.com>