chg: extract gethgcmd logic to a function

gethgcmd is to get original hg (not chg) binary name. This patch extracts
the logic from execcmdserver to make it available for the following patch.

contrib/chg/chg.c

@@ -190,13 +190,22 static void unlockcmdserver(struct cmdse
 	opts->lockfd = -1;
 }
 
-static void execcmdserver(const struct cmdserveropts *opts)
+static const char *gethgcmd(void)
 {
-	const char *hgcmd = getenv("CHGHG");
+	static const char *hgcmd = NULL;
+	if (!hgcmd) {
+		hgcmd = getenv("CHGHG");
 	if (!hgcmd || hgcmd[0] == '\0')
 		hgcmd = getenv("HG");
 	if (!hgcmd || hgcmd[0] == '\0')
 		hgcmd = "hg";
+	}
+	return hgcmd;
+}
+
+static void execcmdserver(const struct cmdserveropts *opts)
+{
+	const char *hgcmd = gethgcmd();
 
 	const char *baseargv[] = {
 		hgcmd,