run-command(win32): resolve the path to the Unix shell early

In 7762975 (Do not use SHELL_PATH from build system in
prepare_shell_cmd on Windows, 2012-04-17), the hard-coded path to the
Unix shell was replaced by passing `sh` instead when executing Unix
shell scripts in Git.

This was done because the hard-coded path to the Unix shell is incorrect
on Windows because it not only is a Unix-style absolute path instead of
a Windows one, but Git uses the runtime prefix feature on Windows, i.e.
the correct path cannot be hard-coded.

Naturally, the `sh` argument will be resolved to the full path of said
executable eventually.

To help fixing the bug where `git var GIT_SHELL_PATH` currently does not
reflect that logic, but shows that incorrect hard-coded Unix-style
absolute path, let's resolve the full path to the `sh` executable early
in the `git_shell_path()` function so that we can use it in `git var`,
too, and be sure that the output is equivalent to what `run_command()`
does when it is asked to execute a command-line using a Unix shell.

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

run-command.c

@@ -274,12 +274,14 @@ int sane_execvp(const char *file, char * const argv[])
 	return -1;
 }
 
-static const char *git_shell_path(void)
+static char *git_shell_path(void)
 {
 #ifndef GIT_WINDOWS_NATIVE
-	return SHELL_PATH;
+	return xstrdup(SHELL_PATH);
 #else
-	return "sh";
+	char *p = locate_in_PATH("sh");
+	convert_slashes(p);
+	return p;
 #endif
 }
 
@@ -289,7 +291,7 @@ static const char **prepare_shell_cmd(struct strvec *out, const char **argv)
 		BUG("shell command is empty");
 
 	if (strcspn(argv[0], "|&;<>()$`\\\"' \t\n*?[#~=%") != strlen(argv[0])) {
-		strvec_push(out, git_shell_path());
+		strvec_push_nodup(out, git_shell_path());
 		strvec_push(out, "-c");
 
 		/*