Assorted fixes around running tests with "-x" tracing option.
* jk/test-suite-tracing:
t/Makefile: introduce TEST_SHELL_PATH
test-lib: make "-x" work with "--verbose-log"
t5615: avoid re-using descriptor 4
test-lib: silence "-x" cleanup under bash
# to say "export LESS=FRX (and LV=-c) if the environment variable
# LESS (and LV) is not set, respectively".
#
+ # Define TEST_SHELL_PATH if you want to use a shell besides SHELL_PATH for
+ # running the test scripts (e.g., bash has better support for "set -x"
+ # tracing).
++#
+# When cross-compiling, define HOST_CPU as the canonical name of the CPU on
+# which the built Git will run (for instance "x86_64").
GIT-VERSION-FILE: FORCE
@$(SHELL_PATH) ./GIT-VERSION-GEN