config: require lowercase for branch.*.autosetupmerge

Although we only documented that branch.*.autosetupmerge would accept
"always" as a value, the actual implementation would accept any
combination of upper- or lower-case. Fix this to be consistent with
documentation and with other values of this config variable.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1556,7 +1556,7 @@ static int git_default_i18n_config(const char *var, const char *value)
 static int git_default_branch_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "branch.autosetupmerge")) {
-		if (value && !strcasecmp(value, "always")) {
+		if (value && !strcmp(value, "always")) {
 			git_branch_track = BRANCH_TRACK_ALWAYS;
 			return 0;
 		} else if (value && !strcmp(value, "inherit")) {