Merge branch 'test-git-installed'

This patch series contains a couple of fixes revolving around testing
an installed Git, via GIT_TEST_INSTALLED=/path/to/git.

The original motivation for these patches is that Git for Windows wants
to provide a version where the Unix shell scripts are interpreted by
BusyBox (to reduce the footprint on disk, mainly), and we want to verify
that this actually works, and is not perchance missing any Unix shell
tool that is present in the Git for Windows SDK but is missing from the
installed set of files.

While the BusyBox-based Git for Windows is not ready for prime time,
this here patch series is, and might be useful for packagers who want to
verify a similar scenario.

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

t/test-lib.sh

@@ -1154,7 +1154,7 @@ test -d "$GIT_BUILD_DIR"/templates/blt || {
 	error "You haven't built things yet, have you?"
 }
 
-if ! test -x "$GIT_BUILD_DIR"/t/helper/test-tool
+if ! test -x "$GIT_BUILD_DIR"/t/helper/test-tool$X
 then
 	echo >&2 'You need to build test-tool:'
 	echo >&2 'Run "make t/helper/test-tool" in the source (toplevel) directory'