doc: change environment variables format

This change GIT_* variables that where in italic style to monospaced font
according to the guideline. It was obtained with

	perl -pi -e "s/\'(GIT_.*?)\'/\`\1\`/g" *.txt

One of the main purposes is to stick to the CodingGuidelines as possible so
that people writting new documentation by mimicking the existing are more likely
to have it right (even if they didn't read the CodingGuidelines).

Signed-off-by: Tom Russello <tom.russello@grenoble-inp.org>
Signed-off-by: Erwan Mathoniere <erwan.mathoniere@grenoble-inp.org>
Signed-off-by: Samuel Groot <samuel.groot@grenoble-inp.org>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/merge-config.txt b/Documentation/merge-config.txt
index 002ca58..df3ea37 100644
--- a/Documentation/merge-config.txt
+++ b/Documentation/merge-config.txt
@@ -61,7 +61,7 @@
 	message if conflicts were detected. Level 1 outputs only
 	conflicts, 2 outputs conflicts and file changes.  Level 5 and
 	above outputs debugging information.  The default is level 2.
-	Can be overridden by the 'GIT_MERGE_VERBOSITY' environment variable.
+	Can be overridden by the `GIT_MERGE_VERBOSITY` environment variable.
 
 merge.<driver>.name::
 	Defines a human-readable name for a custom low-level