config: provide access to the current line number

Users are nowadays trained to see message from CLI tools in the form

    <file>:<lno>: …

To be able to give such messages when notifying the user about
configurations in any config file, it is currently only possible to get
the file name (if the value originates from a file to begin with) via
`current_config_name()`. Now it is also possible to query the current line
number for the configuration.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1308-config-set.sh

@@ -238,8 +238,8 @@ test_expect_success 'error on modifying repo config without repo' '
 
 cmdline_config="'foo.bar=from-cmdline'"
 test_expect_success 'iteration shows correct origins' '
-	echo "[foo]bar = from-repo" >.git/config &&
-	echo "[foo]bar = from-home" >.gitconfig &&
+	printf "[ignore]\n\tthis = please\n[foo]bar = from-repo\n" >.git/config &&
+	printf "[foo]\n\tbar = from-home\n" >.gitconfig &&
 	if test_have_prereq MINGW
 	then
 		# Use Windows path (i.e. *not* $HOME)
@@ -253,18 +253,28 @@ test_expect_success 'iteration shows correct origins' '
 	value=from-home
 	origin=file
 	name=$HOME_GITCONFIG
+	lno=2
 	scope=global
 
+	key=ignore.this
+	value=please
+	origin=file
+	name=.git/config
+	lno=2
+	scope=local
+
 	key=foo.bar
 	value=from-repo
 	origin=file
 	name=.git/config
+	lno=3
 	scope=local
 
 	key=foo.bar
 	value=from-cmdline
 	origin=command line
 	name=
+	lno=-1
 	scope=command
 	EOF
 	GIT_CONFIG_PARAMETERS=$cmdline_config test-tool config iterate >actual &&