Merge branch 'mb/config-document-include' into maint

Add missing documentation for "include" and "includeIf" features in
"git config" file format, which incidentally teaches the command
line completion to include them in its offerings.
source: <pull.1285.v2.git.1658002423864.gitgitgadget@gmail.com>

* mb/config-document-include:
  config.txt: document include, includeIf

Documentation/config.txt

@@ -445,6 +445,8 @@ 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

@@ -2485,6 +2485,13 @@ 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
@@ -2493,6 +2500,12 @@ 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