Fix Lexer::getLongString prefix pruning logic

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)