config: add 'git_config_string' to refactor string config variables.

In many places we just check if a value from the config file is not
NULL, then we duplicate it and return 0. This patch introduces the new
'git_config_string' function to do that.

This function is also used to refactor some code in 'config.c'.
Refactoring other files is left for other patches.

Also not all the code in "config.c" is refactored, because the function
takes a "const char **" as its first parameter, but in many places a
"char *" is used instead of a "const char *". (And C does not allow
using a "char **" instead of a "const char **" without a warning.)

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -584,6 +584,7 @@ extern int git_parse_ulong(const char *, unsigned long *);
 extern int git_config_int(const char *, const char *);
 extern unsigned long git_config_ulong(const char *, const char *);
 extern int git_config_bool(const char *, const char *);
+extern int git_config_string(const char **, const char *, const char *);
 extern int git_config_set(const char *, const char *);
 extern int git_config_set_multivar(const char *, const char *, const char *, int);
 extern int git_config_rename_section(const char *, const char *);

config.c

@@ -309,6 +309,14 @@ int git_config_bool(const char *name, const char *value)
 	return git_config_int(name, value) != 0;
 }
 
+int git_config_string(const char **dest, const char *var, const char *value)
+{
+	if (!value)
+		return config_error_nonbool(var);
+	*dest = xstrdup(value);
+	return 0;
+}
+
 int git_default_config(const char *var, const char *value)
 {
 	/* This needs a better name */
@@ -421,20 +429,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
-	if (!strcmp(var, "i18n.commitencoding")) {
-		if (!value)
-			return config_error_nonbool(var);
-		git_commit_encoding = xstrdup(value);
-		return 0;
-	}
-
-	if (!strcmp(var, "i18n.logoutputencoding")) {
-		if (!value)
-			return config_error_nonbool(var);
-		git_log_output_encoding = xstrdup(value);
-		return 0;
-	}
+	if (!strcmp(var, "i18n.commitencoding"))
+		return git_config_string(&git_commit_encoding, var, value);
 
+	if (!strcmp(var, "i18n.logoutputencoding"))
+		return git_config_string(&git_log_output_encoding, var, value);
 
 	if (!strcmp(var, "pager.color") || !strcmp(var, "color.pager")) {
 		pager_use_color = git_config_bool(var,value);