libbb: avoid problems with quoted characters in tab completion

Commit 9e341902d (libbb: copy entire match during tab-completion)
adjusted the display of matches during tab completion to make any
changes in case visible.

Unfortunately the presence of quoted special characters upsets
the display:

  $ touch t+1+2+3 t+1+4+5
  $ ls t+1<TAB>

changes the command to:

  $ lst\+1\+

In such cases just revert to the upstream code which only displays
the tail of the match, giving:

  $ ls t+1\+

1 files changed, 11 insertions, 3 deletions

diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index 27c1f3a74..4802b46b4 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -1249,6 +1249,9 @@ static NOINLINE void input_tab(smallint *lastWasTab)
         size_t len_found;
         /* Length of string used for matching */
         unsigned match_pfx_len = match_pfx_len;
+#if ENABLE_PLATFORM_MINGW32
+        unsigned orig_pfx_len;
+#endif
         int find_type;
 # if ENABLE_UNICODE_SUPPORT
         /* cursor pos in command converted to multibyte form */
@@ -1313,6 +1316,9 @@ static NOINLINE void input_tab(smallint *lastWasTab)
         {
                 const char *e = match_buf + strlen(match_buf);
                 const char *s = e - match_pfx_len;
+#if ENABLE_PLATFORM_MINGW32
+                orig_pfx_len = match_pfx_len;
+#endif
                 while (s < e)
                         if (is_special_char(*s++))
                                 match_pfx_len++;
@@ -1388,10 +1394,12 @@ static NOINLINE void input_tab(smallint *lastWasTab)
                 strcpy(match_buf, &command_ps[cursor]);
                 /* add match and tail */
 #if ENABLE_PLATFORM_MINGW32
-                sprintf(&command_ps[cursor-match_pfx_len], "%s%s", chosen_match, match_buf);
-#else
-                sprintf(&command_ps[cursor], "%s%s", chosen_match + match_pfx_len, match_buf);
+                if (match_pfx_len == orig_pfx_len) {
+                        sprintf(&command_ps[cursor-match_pfx_len], "%s%s",
+                                        chosen_match, match_buf);
+                } else
 #endif
+                sprintf(&command_ps[cursor], "%s%s", chosen_match + match_pfx_len, match_buf);
                 command_len = strlen(command_ps);
                 /* new pos */
                 pos = cursor + len_found - match_pfx_len;