chg: populate CHGHG if not set

Normally, chg determines which `hg` executable to use by first consulting the
`$CHGHG` and `$HG` environment variables, and if neither are present defaults
to the `hg` found in the user's `$PATH`. If built with the `HGPATHREL` compiler
flag, chg will instead assume that there exists an `hg` executable in the same
directory as the `chg` binary and attempt to use that.

This can cause problems in situations where there are multiple actively-used
Mercurial installations on the same system. When a `chg` client connects to a
running command server, the server process performs some basic validation to
determine whether a new command server needs to be spawned. These checks include
things like checking certain "sensitive" environment variables and config
sections, as well as checking whether the mtime of the extensions, hg's
`__version__.py` module, and the Python interpreter have changed.

Crucially, the command server doesn't explicitly check whether the executable it
is running from matches the executable that the `chg` client would have
otherwise invoked had there been no existing command server process. Without
`HGPATHREL`, this still gets implicitly checked during the validation step,
because the only way to specify an alternate hg executable (apart from `$PATH`)
is via the `$CHGHG` and `$HG` environment variables, both of which are checked.

With `HGPATHREL`, however, the command server has no way of knowing which hg
executable the client would have run. This means that a client located at
`/version_B/bin/chg` will happily connect to a command server running
`/version_A/bin/hg` instead of `/version_B/bin/hg` as expected. A simple
solution is to have the client set `$CHGHG` itself, which then allows the
command server's environment validation to work as intended.

I have tested this manually using two locally built hg installations and it
seems to work with no ill effects. That said, I'm not sure how to write an
automated test for this since the `chg` available to the tests isn't even built
with the `HGPATHREL` compiler flag to begin with.

contrib/chg/chg.c

@@ -232,12 +232,17 b' static const char *gethgcmd(void)'
 			hgcmd = "hg";
 #endif
 	}
+	/* Set $CHGHG to the path to the seleted hg executable if it wasn't
+	 * already set. This has the effect of ensuring that a new command
+	 * server will be spawned if the existing command server is running from
+	 * an executable at a different path. */
+	if (setenv("CHGHG", hgcmd, 1) != 0)
+		abortmsgerrno("failed to setenv");
 	return hgcmd;
 }
 
-static void execcmdserver(const struct cmdserveropts *opts)
+static void execcmdserver(const char *hgcmd, const struct cmdserveropts *opts)
 {
-	const char *hgcmd = gethgcmd();
 
 	const char *baseargv[] = {
 	    hgcmd,     "serve",     "--no-profile",     "--cmdserver",
@@ -375,11 +380,16 b' static hgclient_t *connectcmdserver(stru'
 
 	debugmsg("start cmdserver at %s", opts->initsockname);
 
+	/* Get the path to the hg executable before we fork because this
+	 * function might update the environment, and we want this to be
+	 * reflected in both the parent and child processes. */
+	const char *hgcmd = gethgcmd();
+
 	pid_t pid = fork();
 	if (pid < 0)
 		abortmsg("failed to fork cmdserver process");
 	if (pid == 0) {
-		execcmdserver(opts);
+		execcmdserver(hgcmd, opts);
 	} else {
 		hgc = retryconnectcmdserver(opts, pid);
 	}
@@ -484,7 +494,7 b' static void execoriginalhg(const char *a'
 		abortmsgerrno("failed to exec original hg");
 }
 
-int main(int argc, const char *argv[], const char *envp[])
+int main(int argc, const char *argv[])
 {
 	if (getenv("CHGDEBUG"))
 		enabledebugmsg();
@@ -519,7 +529,10 b' int main(int argc, const char *argv[], c'
 		hgc = connectcmdserver(&opts);
 		if (!hgc)
 			abortmsg("cannot open hg client");
-		hgc_setenv(hgc, envp);
+		/* Use `environ(7)` instead of the optional `envp` argument to
+		 * `main` because `envp` does not update when the environment
+		 * changes, but `environ` does. */
+		hgc_setenv(hgc, (const char *const *)environ);
 		const char **insts = hgc_validate(hgc, argv + 1, argc - 1);
 		int needreconnect = runinstructions(&opts, insts);
 		free(insts);