tests: handle "funny" exit code 127 produced by MSVC-compiled exes

The exit code 127 is well-documented to mean: command not found.

Unfortunately, it is also used as fall-back in Cygwin's
`pinfo::status_exit()` method (which maps things like Windows'
`STATUS_ACCESS_VIOLATION` to `128 | SIGSEGV`).

This is particularly unfortunate because there is no explicit mapping
for `STATUS_STACK_OVERFLOW`. Meaning: when MSVC-compiled executables
produce a stack overflow the exit code in the Cygwin Bash will be 127.
Consequently, the same will be true for the MSYS2 Bash that is used by
Git for Windows.

Now, `jk/tree-name-and-depth-limit` introduces a pair of test cases that
expect a command that produces a stack overflow to fail, which it
typically does with exit code 139 (which means SIGSEGV).

But since MSVC-compiled `git.exe` exits with `STATUS_STACK_OVERFLOW`
which the MSYS2 runtime maps to 127, and since 127 is taken to mean
"command not found" by `test_must_fail`, even though everything works as
planned the two new test cases fail when run in `win+VS test`.

Let's work around this by:

1) recording which C compiler was used, and

2) adding an MSVC-only exception to `test_must_fail` to treat 127 as a
   regular failure.

There is a slight downside of this approach in that a real missing
command could be mistaken for a failure. However, this would be caught
on other platforms, and besides, we use `test_must_fail` only for `git`
and `scalar` anymore, and we can be pretty certain that both are there.

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

contrib/buildsystems/CMakeLists.txt

@@ -1057,7 +1057,8 @@ if(NOT PYTHON_TESTS)
 	set(NO_PYTHON 1)
 endif()
 
-file(WRITE ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "SHELL_PATH='${SHELL_PATH}'\n")
+file(WRITE ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "CMAKE_C_COMPILER='${CMAKE_C_COMPILER}'\n")
+file(APPEND ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "SHELL_PATH='${SHELL_PATH}'\n")
 file(APPEND ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "TEST_SHELL_PATH='${TEST_SHELL_PATH}'\n")
 file(APPEND ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "PERL_PATH='${PERL_PATH}'\n")
 file(APPEND ${CMAKE_BINARY_DIR}/GIT-BUILD-OPTIONS "DIFF='${DIFF}'\n")

t/test-lib-functions.sh

@@ -1112,6 +1112,9 @@ test_must_fail () {
 		return 1
 	elif test $exit_code -eq 127
 	then
+		# Work-around for MSVC-compiled executables
+		case "$CMAKE_C_COMPILER" in *MSVC*) return 0;; esac
+
 		echo >&4 "test_must_fail: command not found: $*"
 		return 1
 	elif test $exit_code -eq 126