pager: factor out a helper to prepare a child process to run the pager

When running a pager, we need to run the program git_pager() gave
us, but we need to make sure we spawn it via the shell (i.e. it is
valid to say PAGER='less -S', for example) and give default values
to $LESS and $LV environment variables.  Factor out these details
to a separate helper function.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -210,7 +210,9 @@ struct cache_entry {
 #error "CE_EXTENDED_FLAGS out of range"
 #endif
 
+/* Forward structure decls */
 struct pathspec;
+struct child_process;
 
 /*
  * Copy the sha1 and stat state of a cache entry from one to
@@ -1550,6 +1552,7 @@ extern int pager_use_color;
 extern int term_columns(void);
 extern int decimal_width(uintmax_t);
 extern int check_pager_config(const char *cmd);
+extern void prepare_pager_args(struct child_process *, const char *pager);
 
 extern const char *editor_program;
 extern const char *askpass_program;

pager.c

@@ -53,6 +53,16 @@ const char *git_pager(int stdout_is_tty)
 	return pager;
 }
 
+void prepare_pager_args(struct child_process *pager_process, const char *pager)
+{
+	argv_array_push(&pager_process->args, pager);
+	pager_process->use_shell = 1;
+	if (!getenv("LESS"))
+		argv_array_push(&pager_process->env_array, "LESS=FRX");
+	if (!getenv("LV"))
+		argv_array_push(&pager_process->env_array, "LV=-c");
+}
+
 void setup_pager(void)
 {
 	const char *pager = git_pager(isatty(1));
@@ -69,13 +79,8 @@ void setup_pager(void)
 	setenv("GIT_PAGER_IN_USE", "true", 1);
 
 	/* spawn the pager */
-	argv_array_push(&pager_process.args, pager);
-	pager_process.use_shell = 1;
+	prepare_pager_args(&pager_process, pager);
 	pager_process.in = -1;
-	if (!getenv("LESS"))
-		argv_array_push(&pager_process.env_array, "LESS=FRX");
-	if (!getenv("LV"))
-		argv_array_push(&pager_process.env_array, "LV=-c");
 	argv_array_push(&pager_process.env_array, "GIT_PAGER_IN_USE");
 	if (start_command(&pager_process))
 		return;