[LyX/2.3.x] Fix Lexer::getLongString prefix pruning logic

[Jean-Marc Lasgouttes]

commit cc2835392294fb1e82a89b2ec3803f78c9183f18
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Wed Aug 23 12:25:29 2017 +0200

    Fix Lexer::getLongString prefix pruning logic
    
    This fixes 3 bugs:
    1/ only spaces are considered as part of the prefix
    2/ leading tabs are removed unconditionally
    3/ off-by-one error in the removal
    
    The new intended behavior is:
    
    1/ find the prefix (sequence of spaces and tabs) before the first
       line; remove this prefix from the line
    2/ on the next lines, check whether they start with the same prefix,
      and if they do, strip this prefix
    
    (cherry picked from commit 48f099d93ab179eef76594ced6b735aa912c03f6)
---
 src/Lexer.cpp |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/Lexer.cpp b/src/Lexer.cpp
index becd157..098faa9 100644
--- a/src/Lexer.cpp
+++ b/src/Lexer.cpp
@@ -738,7 +738,7 @@ docstring Lexer::getLongString(docstring const & endtoken)
                        break;
 
                if (firstline) {
-                       size_t i = tmpstr.find_first_not_of(char_type(' '));
+                       size_t i = tmpstr.find_first_not_of(from_ascii(" \t"));
                        if (i != string::npos)
                                prefix = tmpstr.substr(0, i);
                        firstline = false;
@@ -747,10 +747,10 @@ docstring Lexer::getLongString(docstring const & endtoken)
 
                // further lines in long strings may have the same
                // whitespace prefix as the first line. Remove it.
-               if (prefix.length() && prefixIs(tmpstr, prefix))
-                       tmpstr.erase(0, prefix.length() - 1);
+               if (!prefix.empty() && prefixIs(tmpstr, prefix))
+                       tmpstr.erase(0, prefix.length());
 
-               str += ltrim(tmpstr, "\t") + '\n';
+               str += tmpstr + '\n';
        }
 
        if (!pimpl_->is)