Merge branch 'bw/config-h'

This backports f31d23a399 (Merge branch 'bw/config-h', 2017-06-24) from
upstream Git's `master` branch, plus a patch that seems to be required
to let the test suite pass.

This topic branch fixes problems when looking up aliases in worktrees.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -8,6 +8,7 @@
 #include "../cache.h"
 #include "win32/lazyload.h"
 #include "win32/exit-process.h"
+#include "../config.h"
 
 #define HCAST(type, handle) ((type)(intptr_t)handle)
 

config.c

@@ -1532,6 +1532,8 @@ static int do_git_config_sequence(const struct config_options *opts,
 
 	if (opts->commondir)
 		repo_config = mkpathdup("%s/config", opts->commondir);
+	else if (opts->git_dir)
+		repo_config = mkpathdup("%s/config", opts->git_dir);
 	else
 		repo_config = NULL;