Getting ready for 2.12.1

So far I've merged the following topics to 'maint' in preparation
for the first maintenance update for 2.12:

 * Reduce authentication round-trip over HTTP when the server supports
   just a single authentication method.  This also improves the
   behaviour when Git is misconfigured to enable http.emptyAuth
   against a server that does not authenticate without a username
   (i.e. not using Kerberos etc., which makes http.emptyAuth
   pointless).

 * Windows port wants to use OpenSSL's implementation of SHA-1
   routines, so let them.

 * Add 32-bit Linux variant to the set of platforms to be tested with
   Travis CI.

 * When a redirected http transport gets an error during the
   redirected request, we ignored the error we got from the server,
   and ended up giving a not-so-useful error message.

 * The patch subcommand of "git add -i" was meant to have paths
   selection prompt just like other subcommand, unlike "git add -p"
   directly jumps to hunk selection.  Recently, this was broken and
   "add -i" lost the paths selection dialog, but it now has been
   fixed.

 * Git v2.12 was shipped with an embarrassing breakage where various
   operations that verify paths given from the user stopped dying when
   seeing an issue, and instead later triggering segfault.

 * The code to parse "git log -L..." command line was buggy when there
   are many ranges specified with -L; overrun of the allocated buffer
   has been fixed.

 * The command-line parsing of "git log -L" copied internal data
   structures using incorrect size on ILP32 systems.

I think travis will try to build it tonight on our usual suspect
platforms; it would be appreciated if those who are testing for
their favourite platform to also give it a whirl and report if there
are any breakages.

There are other "fix" topics that have been merged to 'master' that
eventually could go to 'maint'; as I am inclined to keep the first
one to the minimum, they will be merged and shipped in 2.12.2 or
later.

Thanks.