mingw: respect core.shell when executing scripts

On Windows, we have to emulate that Linux/Unix/macOS feature where a
file starting with a `#!` line and being marked as executable is run as
a script through the interpreter specified by said `#!` line.

Traditionally, we ignore the actual path specified in that line because
it will be a Unix-style path anyway, something that `git.exe` is not
even supposed to understand. We then go on to look up the actual path of
the interpreter by iterating over the components in the environment
variable `PATH`.

Let's special-case `sh` in that scenario when the config setting
`core.shell` exists: in this case, we want to use it instead.

This allows us to configure BusyBox' `ash` to be used for all of the
shell scripting needs of the BusyBox flavor of MinGit.

While at it, assume that any shell configured via `core.shell` is _not_
an MSYS2 shell, i.e. that we should use regular Win32 command-line
quoting, not MSYS2/Cygwin one.

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

compat/mingw.c

@@ -1277,6 +1277,10 @@ static char *path_lookup(const char *cmd, int exe_only)
 	if (strpbrk(cmd, "/\\"))
 		return xstrdup(cmd);
 
+	if (!strcmp(cmd, "sh") &&
+	    (prog = xstrdup_or_null(get_shell_path(NULL))))
+		return prog;
+
 	path = mingw_getenv("PATH");
 	if (!path)
 		return NULL;
@@ -1463,6 +1467,12 @@ static int is_msys2_sh(const char *cmd)
 		if (ret >= 0)
 			return ret;
 
+		if (get_shell_path(NULL)) {
+			/* Assume an overridden shell is not MSYS2 */
+			ret = 0;
+			return ret;
+		}
+
 		p = path_lookup(cmd, 0);
 		if (!p)
 			ret = 0;