manpages: italicize gitk's name (where it was in teletype font)

The name `gitk` is sometimes meant to be entered at the command
prompt, but most uses are just referring to the program with that
name (not the incantation to start it).

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

Documentation/git-bisect.txt

@@ -98,7 +98,7 @@ During the bisection process, you can say
 $ git bisect visualize
 ------------
 
-to see the currently remaining suspects in `gitk`.  `visualize` is a bit
+to see the currently remaining suspects in 'gitk'.  `visualize` is a bit
 too long to type and `view` is provided as a synonym.
 
 If 'DISPLAY' environment variable is not set, 'git-log' is used

Documentation/git-gui.txt

@@ -16,9 +16,9 @@ on allowing users to make changes to their repository by making
 new commits, amending existing ones, creating branches, performing
 local merges, and fetching/pushing to remote repositories.
 
-Unlike `gitk`, 'git-gui' focuses on commit generation
+Unlike 'gitk', 'git-gui' focuses on commit generation
 and single file annotation and does not show project history.
-It does however supply menu actions to start a `gitk` session from
+It does however supply menu actions to start a 'gitk' session from
 within 'git-gui'.
 
 'git-gui' is known to work on all popular UNIX systems, Mac OS X,

Documentation/gitcore-tutorial.txt

@@ -878,7 +878,7 @@ means: normally it will just show you your current `HEAD`) and their
 histories. You can also see exactly how they came to be from a common
 source.
 
-Anyway, let's exit `gitk` (`^Q` or the File menu), and decide that we want
+Anyway, let's exit 'gitk' (`^Q` or the File menu), and decide that we want
 to merge the work we did on the `mybranch` branch into the `master`
 branch (which is currently our `HEAD` too). To do that, there's a nice
 script called 'git-merge', which wants to know which branches you want

Documentation/gittutorial.txt

@@ -529,7 +529,7 @@ then merged back together, the order in which 'git-log' presents
 those commits is meaningless.
 
 Most projects with multiple contributors (such as the linux kernel,
-or git itself) have frequent merges, and `gitk` does a better job of
+or git itself) have frequent merges, and 'gitk' does a better job of
 visualizing their history.  For example,
 
 -------------------------------------