grep: add grep.fullName config variable

This configuration variable sets the default for the --full-name option.
Signed-off-by: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -53,6 +53,9 @@ grep.extendedRegexp::
 	option is ignored when the 'grep.patternType' option is set to a value
 	other than 'default'.
 
+grep.fullName::
+	If set to true, enable '--full-name' option by default.
+
 
 OPTIONS
 -------

grep.c

@@ -86,6 +86,11 @@ int grep_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "grep.fullname")) {
+		opt->relative = !git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "color.grep"))
 		opt->color = git_config_colorbool(var, value);
 	else if (!strcmp(var, "color.grep.context"))