var(win32): do report the GIT_SHELL_PATH that is actually used

On Windows, Unix-like paths like `/bin/sh` make very little sense. In
the best case, they simply don't work, in the worst case they are
misinterpreted as absolute paths that are relative to the drive
associated with the current directory.

To that end, Git does not actually use the path `/bin/sh` that is
recorded e.g. in Unix shell scripts' hash-bang lines. Instead, as of
7762975 (Do not use SHELL_PATH from build system in prepare_shell_cmd
on Windows, 2012-04-17), it re-interprets `/bin/sh` as "look up `sh` on
the `PATH` and use the result instead".

However, when 1e65721 (var: add support for listing the shell,
2023-06-27) introduced support for `git var GIT_SHELL_PATH`, Windows was
not special-cased as above, which is why it outputs `/bin/sh` even
though that disagrees with what Git actually uses.

Let's fix this, and also adjust the corresponding test case to verify
that it actually finds a working executable.

Reported-by: Phillip Wood <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

builtin/var.c

@@ -51,7 +51,13 @@ static char *default_branch(int ident_flag UNUSED)
 
 static char *shell_path(int ident_flag UNUSED)
 {
+#ifdef WIN32
+	char *p = locate_in_PATH("sh");
+	convert_slashes(p);
+	return p;
+#else
 	return xstrdup(SHELL_PATH);
+#endif
 }
 
 static char *git_attr_val_system(int ident_flag UNUSED)

t/t0007-git-var.sh

@@ -157,7 +157,7 @@ test_expect_success POSIXPERM 'GIT_SHELL_PATH points to a valid executable' '
 test_expect_success MINGW 'GIT_SHELL_PATH points to a suitable shell' '
 	shellpath=$(git var GIT_SHELL_PATH) &&
 	case "$shellpath" in
-	*sh) ;;
+	[A-Z]:/*/sh.exe) test -f "$shellpath";;
 	*) return 1;;
 	esac
 '