io: change prompt after line continuation

This ensures that line continuations inside of e.g. integer and float
expressions properly show a line continuation prompt.

Fixes #1992

src/io.c

@@ -201,8 +201,14 @@ Char GET_NEXT_CHAR(void)
     while (*STATE(In) == '\\' || *STATE(In) == 0) {
         if (!*STATE(In))
             GetLine();
-        else if (STATE(In)[1] == '\n')
+        else if (STATE(In)[1] == '\n') {
             STATE(In) += 2;
+            // print only a partial prompt from now on
+            if (!SyQuiet)
+                STATE(Prompt) = "> ";
+            else
+                STATE(Prompt) = "";
+        }
         else if (STATE(In)[1] == '\r')
             STATE(In) += (STATE(In)[2] == '\n') ? 3 : 2;
         else

tst/testinstall/linecontinuation.tst

@@ -35,6 +35,19 @@ gap> {x.\
 > ..}->x;
 function( x... ) ... end
 
+# inside float expressions
+gap> 1.2e\
+> 0;
+1.2
+gap> 1.1\
+> ;
+1.1
+
+# inside integer expressions
+gap> 12\
+> 3;
+123
+
 # however, in comments, you cannot use line continuations:
 gap> # 1234\
 gap> 5;