Add an option to specify a file to config builtin

There are (really!) systems where using environment variables is very
cumbersome (yes, Windows, it has problems unsetting them). Besides this
form is shorter.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-config.txt b/Documentation/git-config.txt
index 5f66a7f..88acf6c 100644
--- a/Documentation/git-config.txt
+++ b/Documentation/git-config.txt
@@ -9,17 +9,17 @@
 SYNOPSIS
 --------
 [verse]
-'git-config' [--system | --global] [type] [-z|--null] name [value [value_regex]]
-'git-config' [--system | --global] [type] --add name value
-'git-config' [--system | --global] [type] --replace-all name [value [value_regex]]
-'git-config' [--system | --global] [type] [-z|--null] --get name [value_regex]
-'git-config' [--system | --global] [type] [-z|--null] --get-all name [value_regex]
-'git-config' [--system | --global] [type] [-z|--null] --get-regexp name_regex [value_regex]
-'git-config' [--system | --global] --unset name [value_regex]
-'git-config' [--system | --global] --unset-all name [value_regex]
-'git-config' [--system | --global] --rename-section old_name new_name
-'git-config' [--system | --global] --remove-section name
-'git-config' [--system | --global] [-z|--null] -l | --list
+'git-config' [--system | --global | [-f|--file] config-file] [type] [-z|--null] name [value [value_regex]]
+'git-config' [--system | --global | [-f|--file] config-file] [type] --add name value
+'git-config' [--system | --global | [-f|--file] config-file] [type] --replace-all name [value [value_regex]]
+'git-config' [--system | --global | [-f|--file] config-file] [type] [-z|--null] --get name [value_regex]
+'git-config' [--system | --global | [-f|--file] config-file] [type] [-z|--null] --get-all name [value_regex]
+'git-config' [--system | --global | [-f|--file] config-file] [type] [-z|--null] --get-regexp name_regex [value_regex]
+'git-config' [--system | --global | [-f|--file] config-file] --unset name [value_regex]
+'git-config' [--system | --global | [-f|--file] config-file] --unset-all name [value_regex]
+'git-config' [--system | --global | [-f|--file] config-file] --rename-section old_name new_name
+'git-config' [--system | --global | [-f|--file] config-file] --remove-section name
+'git-config' [--system | --global | [-f|--file] config-file] [-z|--null] -l | --list
 
 DESCRIPTION
 -----------
@@ -42,8 +42,8 @@
 
 This command will fail if:
 
-. The .git/config file is invalid,
-. Can not write to .git/config,
+. The config file is invalid,
+. Can not write to the config file,
 . no section was provided,
 . the section or key is invalid,
 . you try to unset an option which does not exist,
@@ -93,6 +93,9 @@
 +
 See also <<FILES>>.
 
+-f config-file, --file config-file::
+	Use the given config file instead of the one specified by GIT_CONFIG.
+
 --remove-section::
 	Remove the given section from the configuration file.