fix repl rsearch with one-char query

The backward i-search needs to set the "start" parameter of rsearch
to the end (minus 1) of the current search window.
This fixes e.g.:
* searching backwards only one character remains stuck on the same match
* searching backwards 'aaa' skips one of the matches in 'aaaa'

base/REPL.jl

@@ -514,12 +514,12 @@ function history_search(hist::REPLHistoryProvider, query_buffer::IOBuffer, respo
         return true
     end
 
-    searchfunc,delta = backwards ? (rsearch,0) : (search,1)
+    searchfunc, searchstart = backwards ? (rsearch, b-1) : (search, a+1)
 
     # Start searching
     # First the current response buffer
-    if 1 <= a+delta <= length(response_str)
-        match = searchfunc(response_str, searchdata, a+delta)
+    if 1 <= searchstart <= length(response_str)
+        match = searchfunc(response_str, searchdata, searchstart)
         if match != 0:-1
             seek(response_buffer, first(match)-1)
             return true

test/repl.jl

@@ -162,6 +162,17 @@ begin
     @test LineEdit.mode(s) == shell_mode
     @test buffercontents(LineEdit.buffer(s)) == "ll"
 
+    # Test that searching backwards with a one-letter query doesn't
+    # return indefinitely the same match.
+    LineEdit.enter_search(s, histp, true)
+    write(ss.query_buffer, "l")
+    LineEdit.update_display_buffer(ss, ss)
+    LineEdit.history_next_result(s, ss)
+    LineEdit.update_display_buffer(ss, ss)
+    LineEdit.accept_result(s, histp)
+    @test LineEdit.mode(s) == repl_mode
+    @test buffercontents(LineEdit.buffer(s)) == "shell"
+
     # Issue #7551
     # Enter search mode and try accepting an empty result
     REPL.history_reset_state(hp)