Change @allow_colons_in_path default value to !@show_path

ie @show_path is on then @allow_colons_in_path is off and vice versa.

Prior to this change @allow_colons_in_path defaulted to off always, but
there's no reason to stop colons in the path if the path isn't shown so
this is a better default.

For example here's a git diff command that prior to this change would
have caused showlinenum to abort by default, but now it's processed:

git diff HEAD~1:./foo/bar HEAD:./foo/baz | showlinenum.awk

README.md

@@ -97,9 +97,9 @@ Here is an example of a removed binary file, path shown:
 `calc.exe:~:`
 
 ### Allow colons in path.
-#### `@allow_colons_in_path [0,1] default: 0`
+#### `@allow_colons_in_path [0,1] default: ( show_path ? 0 : 1 )`
 
-By default this script will abort if it encounters a path that contains a colon. That's done to guarantee that this script's diff line output can always be parsed with the first colon occurring immediately after the full path, if the path is shown. Even if it's not shown it's still checked.
+If this option is off then abort if a path that contains a colon is encountered. That's done to guarantee that this script's diff line output can always be parsed with the first colon occurring immediately after the full path. Note git diff paths may start with '<commit>:' like HEAD:./foo/bar, and for such a path this option would need to be on.
 
 ### Add color to some sections.
 #### `@color_{line_number,path,separator} <num>[;num][;num]`

showlinenum.awk

@@ -139,16 +139,18 @@
 #
 ##
 #
-# @allow_colons_in_path [0,1] default: 0
+# @allow_colons_in_path [0,1] default: ( show_path ? 0 : 1 )
 # Allow colons in path.
 #
-# By default this script will abort if it encounters a path that contains a colon. That's done to
+# If this option is off then abort if a path that contains a colon is encountered. That's done to
 # guarantee that this script's diff line output can always be parsed with the first colon occurring
-# immediately after the full path, if the path is shown. Even if it's not shown it's still checked.
+# immediately after the full path. Note git diff paths may start with '<commit>:' like
+# HEAD:./foo/bar, and for such a path this option would need to be on.
 #
 ##
 #
 # @color_{line_number,path,separator} <num>[;num][;num]
+# Add color to some sections.
 #
 # Color the respective section using one or more ANSI color codes.
 # This is not recommended unless you are outputting to the terminal.
@@ -209,7 +211,7 @@ function init()
     show_hunk = get_bool( show_hunk, ( show_header ? 1 : 0 ) );
     show_path = get_bool( show_path, ( show_header ? 0 : 1 ) );
     show_binary = get_bool( show_binary, ( show_path ? 1 : 0 ) );
-    allow_colons_in_path = get_bool( allow_colons_in_path, 0 );
+    allow_colons_in_path = get_bool( allow_colons_in_path, ( show_path ? 0 : 1 ) );
 }
 
 function FATAL( error_message )
@@ -263,9 +265,17 @@ function fix_extracted_path( input )
 
     if( !allow_colons_in_path && ( input ~ /:/ ) )
     {
-        errmsg = "fix_extracted_path(): colons in path are forbidden by default in deference to ";
-        errmsg = errmsg "scripts which may parse this script's output and rely on the colon as a ";
-        errmsg = errmsg "separator. To override use command line option allow_colons_in_path=1.";
+        errmsg = "fix_extracted_path(): colons in path are forbidden ";
+        if(show_path)
+        {
+            errmsg = errmsg "by default when show_path is on in deference to scripts which may ";
+            errmsg = errmsg "parse this script's output and rely on the colon as a separator. To ";
+            errmsg = errmsg "override use command line option allow_colons_in_path=1.";
+        }
+        else
+        {
+            errmsg = errmsg "because allow_colons_in_path is off.";
+        }
         errmsg = errmsg "\n" "Path: " input;
         FATAL( errmsg );
     }