add, merge, diff: do not use strcasecmp to compare config variable names

The config machinery already makes section and variable names
lowercase when parsing them, so using strcasecmp for comparison just
feels wasteful.  No noticeable change intended.

Noticed-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -330,8 +330,8 @@ static struct option builtin_add_options[] = {
 
 static int add_config(const char *var, const char *value, void *cb)
 {
-	if (!strcasecmp(var, "add.ignoreerrors") ||
-	    !strcasecmp(var, "add.ignore-errors")) {
+	if (!strcmp(var, "add.ignoreerrors") ||
+	    !strcmp(var, "add.ignore-errors")) {
 		ignore_add_errors = git_config_bool(var, value);
 		return 0;
 	}

merge-recursive.c

@@ -1711,15 +1711,15 @@ int merge_recursive_generic(struct merge_options *o,
 static int merge_recursive_config(const char *var, const char *value, void *cb)
 {
 	struct merge_options *o = cb;
-	if (!strcasecmp(var, "merge.verbosity")) {
+	if (!strcmp(var, "merge.verbosity")) {
 		o->verbosity = git_config_int(var, value);
 		return 0;
 	}
-	if (!strcasecmp(var, "diff.renamelimit")) {
+	if (!strcmp(var, "diff.renamelimit")) {
 		o->diff_rename_limit = git_config_int(var, value);
 		return 0;
 	}
-	if (!strcasecmp(var, "merge.renamelimit")) {
+	if (!strcmp(var, "merge.renamelimit")) {
 		o->merge_rename_limit = git_config_int(var, value);
 		return 0;
 	}

xdiff-interface.c

@@ -347,7 +347,7 @@ int git_xmerge_style = -1;
 
 int git_xmerge_config(const char *var, const char *value, void *cb)
 {
-	if (!strcasecmp(var, "merge.conflictstyle")) {
+	if (!strcmp(var, "merge.conflictstyle")) {
 		if (!value)
 			die("'%s' is not a boolean", var);
 		if (!strcmp(value, "diff3"))