run-command: declare the git_shell_path() function globally

The intention is to use it in `git var GIT_SHELL_PATH`, therefore we
need this function to stop being file-local only.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.h

@@ -195,6 +195,11 @@ int is_executable(const char *name);
  */
 int exists_in_PATH(const char *command);
 
+/**
+ * Return the path that is used to execute Unix shell command-lines.
+ */
+char *git_shell_path(void);
+
 /**
  * Start a sub-process. Takes a pointer to a `struct child_process`
  * that specifies the details and returns pipe FDs (if requested).