Documentation: tiny git config manual tweaks

As a verb, 'setup' is spelled 'set up'.  “diff commands such as
diff-files” scans better without a comma.  Clarify that shallow
and deep are special non-boolean values for format.thread rather
than boolean values with some other name.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

Documentation/config.txt

@@ -537,7 +537,7 @@ apply.whitespace::
 	as the '--whitespace' option. See linkgit:git-apply[1].
 
 branch.autosetupmerge::
-	Tells 'git-branch' and 'git-checkout' to setup new branches
+	Tells 'git-branch' and 'git-checkout' to set up new branches
 	so that linkgit:git-pull[1] will appropriately merge from the
 	starting point branch. Note that even if this option is not set,
 	this behavior can be chosen per-branch using the `--track`
@@ -725,7 +725,7 @@ diff.autorefreshindex::
 	contents in the work tree match the contents in the
 	index.  This option defaults to true.  Note that this
 	affects only 'git-diff' Porcelain, and not lower level
-	'diff' commands, such as 'git-diff-files'.
+	'diff' commands such as 'git-diff-files'.
 
 diff.external::
 	If this config variable is set, diff generation is not
@@ -841,8 +841,8 @@ format.pretty::
 
 format.thread::
 	The default threading style for 'git-format-patch'.  Can be
-	either a boolean value, `shallow` or `deep`.  `shallow`
-	threading makes every mail a reply to the head of the series,
+	a boolean value, or `shallow` or `deep`.  `shallow` threading
+	makes every mail a reply to the head of the series,
 	where the head is chosen from the cover letter, the
 	`\--in-reply-to`, and the first patch mail, in this order.
 	`deep` threading makes every mail a reply to the previous one.