Merge branch 'jk/interop-test' into maint

Picking two versions of Git and running tests to make sure the
older one and the newer one interoperate happily has now become
possible.

* jk/interop-test:
t/interop: add test of old clients against modern git-daemon
t: add an interoperability test harness

diff --combined Makefile
index ca9f16d19a969e354990c34987414c849bbf6461,7156b051e7aea7019b573b3633a24dcb5e5f475e..d5454ca33632f8027b06399acc56b7163074b056
--- 1/Makefile
--- 2/Makefile
+++ b/Makefile
@@@ -102,6 -102,8 +102,6 @@@ all:
  #
  # Define MKDIR_WO_TRAILING_SLASH if your mkdir() can't deal with trailing slash.
  #
 -# Define NO_MKSTEMPS if you don't have mkstemps in the C library.
 -#
  # Define NO_GECOS_IN_PWENT if you don't have pw_gecos in struct passwd
  # in the C library.
  #
@@@ -1278,6 -1280,9 +1278,6 @@@ ifdef MKDIR_WO_TRAILING_SLAS
        COMPAT_CFLAGS += -DMKDIR_WO_TRAILING_SLASH
        COMPAT_OBJS += compat/mkdir.o
  endif
 -ifdef NO_MKSTEMPS
 -      COMPAT_CFLAGS += -DNO_MKSTEMPS
 -endif
  ifdef NO_UNSETENV
        COMPAT_CFLAGS += -DNO_UNSETENV
        COMPAT_OBJS += compat/unsetenv.o
@@@ -2249,6 -2254,9 +2249,9 @@@ endi
  ifdef GIT_PERF_MAKE_OPTS
        @echo GIT_PERF_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_OPTS)))'\' >>$@+
  endif
+ ifdef GIT_INTEROP_MAKE_OPTS
+       @echo GIT_INTEROP_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_INTEROP_MAKE_OPTS)))'\' >>$@+
+ endif
  ifdef TEST_GIT_INDEX_VERSION
        @echo TEST_GIT_INDEX_VERSION=\''$(subst ','\'',$(subst ','\'',$(TEST_GIT_INDEX_VERSION)))'\' >>$@+
  endif