Fix typo: existant->existent
Fix typo: existant->existent

This typo was discovered in core git sources.
Clean in it up in git-gui too.
There is just one occurence in a comment line.

Signed-off-by: Dmitry Ivankov <c53dcc58d2ff10b5d06f81320a2d1f01379576d0@gmail.com>
Signed-off-by: Pat Thoyts <fe43a83262eef1680511f524a1e74d43c633cffe@users.sourceforge.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git