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/git-p4.txt b/Documentation/git-p4.txt
index 35e3170..c349276 100644
--- a/Documentation/git-p4.txt
+++ b/Documentation/git-p4.txt
@@ -166,7 +166,7 @@
 All commands except clone accept these options.
 
 --git-dir <dir>::
-	Set the 'GIT_DIR' environment variable.  See linkgit:git[1].
+	Set the `GIT_DIR` environment variable.  See linkgit:git[1].
 
 -v::
 --verbose::