Git v2.6.4 released

Fixes since v2.6.3
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 * The "configure" script did not test for -lpthread correctly, which
   upset some linkers.

 * Add support for talking http/https over socks proxy.

 * Portability fix for Windows, which may rewrite $SHELL variable using
   non-POSIX paths.

 * We now consistently allow all hooks to ignore their standard input,
   rather than having git complain of SIGPIPE.

 * Fix shell quoting in contrib script.

 * Test portability fix for a topic in v2.6.1.

 * Allow tilde-expansion in some http config variables.

 * Give a useful special case "diff/show --word-diff-regex=." as an
   example in the documentation.

 * Fix for a corner case in filter-branch.

 * Make git-p4 work on a detached head.