improve handling of revision string (#630)

we use the same standard revision string as it is common at Snapcraft
the revision in the debug output is suppressed when there are no commits since the last tag

configure.ac

@@ -12,12 +12,17 @@ AC_GNU_SOURCE
 m4_ifndef([PKG_PROG_PKG_CONFIG], [m4_fatal([Please install pkg-config.])])
 PKG_PROG_PKG_CONFIG
 
-AC_PATH_PROG(GIT, [git], [""], "$PATH:/sbin:/usr/sbin")
-AS_IF([test "x$GIT" = "x"], [
-	REVISION=unavailable
-], [
-	REVISION=`git describe --tags | sed -e 's/-/+git/;y/-/./'`
+REVISION=""
+AS_IF([test -d .git], [
+	AC_PATH_PROG(GIT, [git], [""], "$PATH:/sbin:/usr/sbin")
+	AS_IF([test "x$GIT" != "x"], [
+		REVISION=`$GIT describe --tags | sed -e 's/-/+git/;y/-/./'`
+	])
+])
+AS_IF([test "x$REVISION" = "x"], [
+	REVISION="unavailable"
 ])
+
 AC_SUBST(REVISION)
 
 # Helps support multiarch by setting 'host_os' and 'host_cpu'

src/main.c

@@ -293,6 +293,8 @@ int main(int argc, char **argv)
 			if (strcmp(long_options[option_index].name,
 			           "version") == 0) {
 				printf(VERSION "\n");
+				if (strcmp(&REVISION[1], VERSION))
+					log_debug("revision " REVISION "\n");
 				ret = EXIT_SUCCESS;
 				goto exit;
 			}
@@ -518,7 +520,8 @@ int main(int argc, char **argv)
 	log_debug_all("ATTENTION: the output contains sensitive information such as the THE CLEAR TEXT PASSWORD.\n");
 
 	log_debug("openfortivpn " VERSION "\n");
-	log_debug("    revision " REVISION "\n");
+	if (strcmp(&REVISION[1], VERSION))
+		log_debug("revision " REVISION "\n");
 
 	// Load config file
 	if (config_file[0] != '\0') {