config.txt: document include, includeIf

Git config's tab completion does not yet know about the "include"
and "includeIf" sections, nor the related "path" variable.

Add a description for these two sections in
'Documentation/config/includeif.txt', which points to git-config's
documentation, specifically the "Includes" and "Conditional Includes"
subsections.

As a side effect, tab completion can successfully complete the
'include', 'includeIf', and 'include.add' expressions.
This effect is tested by two new ad-hoc tests.
Variable completion only works for "include" for now.

Credit for the ideas behind this patch goes to
Ævar Arnfjörð Bjarmason.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Manuel Boni <ziosombrero@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -410,6 +410,8 @@ include::config/i18n.txt[]
 
 include::config/imap.txt[]
 
+include::config/includeif.txt[]
+
 include::config/index.txt[]
 
 include::config/init.txt[]

Documentation/config/includeif.txt

@@ -0,0 +1,6 @@
+include.path::
+includeIf.<condition>.path::
+	Special variables to include other configuration files. See
+	the "CONFIGURATION FILE" section in the main
+	linkgit:git-config[1] documentation,
+	specifically the "Includes" and "Conditional Includes" subsections.

t/t9902-completion.sh

@@ -2330,6 +2330,13 @@ test_expect_success 'git config - section' '
 	EOF
 '
 
+test_expect_success 'git config - section include, includeIf' '
+	test_completion "git config inclu" <<-\EOF
+	include.Z
+	includeIf.Z
+	EOF
+'
+
 test_expect_success 'git config - variable name' '
 	test_completion "git config log.d" <<-\EOF
 	log.date Z
@@ -2338,6 +2345,12 @@ test_expect_success 'git config - variable name' '
 	EOF
 '
 
+test_expect_success 'git config - variable name include' '
+	test_completion "git config include.p" <<-\EOF
+	include.path Z
+	EOF
+'
+
 test_expect_success 'git config - value' '
 	test_completion "git config color.pager " <<-\EOF
 	false Z