Documentation: make environment variable formatting more consistent

Documentation/git.txt is not consistent in the way it
stylizes mentions of Environment Variables. Most of them are
enclosed in single quotes, some are enclosed in backticks,
some are not enclosed.

Signed-off-by: Andrey Rybak <rybak.a.v@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

Documentation/git.txt

@@ -1056,7 +1056,7 @@ of clones and fetches.
 	cloning of shallow repositories.
 	See 'GIT_TRACE' for available trace output options.
 
-GIT_LITERAL_PATHSPECS::
+'GIT_LITERAL_PATHSPECS'::
 	Setting this variable to `1` will cause Git to treat all
 	pathspecs literally, rather than as glob patterns. For example,
 	running `GIT_LITERAL_PATHSPECS=1 git log -- '*.c'` will search
@@ -1065,15 +1065,15 @@ GIT_LITERAL_PATHSPECS::
 	literal paths to Git (e.g., paths previously given to you by
 	`git ls-tree`, `--raw` diff output, etc).
 
-GIT_GLOB_PATHSPECS::
+'GIT_GLOB_PATHSPECS'::
 	Setting this variable to `1` will cause Git to treat all
 	pathspecs as glob patterns (aka "glob" magic).
 
-GIT_NOGLOB_PATHSPECS::
+'GIT_NOGLOB_PATHSPECS'::
 	Setting this variable to `1` will cause Git to treat all
 	pathspecs as literal (aka "literal" magic).
 
-GIT_ICASE_PATHSPECS::
+'GIT_ICASE_PATHSPECS'::
 	Setting this variable to `1` will cause Git to treat all
 	pathspecs as case-insensitive.
 
@@ -1087,7 +1087,7 @@ GIT_ICASE_PATHSPECS::
 	variable when it is invoked as the top level command by the
 	end user, to be recorded in the body of the reflog.
 
-`GIT_REF_PARANOIA`::
+'GIT_REF_PARANOIA'::
 	If set to `1`, include broken or badly named refs when iterating
 	over lists of refs. In a normal, non-corrupted repository, this
 	does nothing. However, enabling it may help git to detect and
@@ -1098,7 +1098,7 @@ GIT_ICASE_PATHSPECS::
 	an operation has touched every ref (e.g., because you are
 	cloning a repository to make a backup).
 
-`GIT_ALLOW_PROTOCOL`::
+'GIT_ALLOW_PROTOCOL'::
 	If set, provide a colon-separated list of protocols which are
 	allowed to be used with fetch/push/clone. This is useful to
 	restrict recursive submodule initialization from an untrusted