git-gui: Show a round number of bytes of large untracked text files

git-gui: Show a round number of bytes of large untracked text files

If an untracked text file is selected, then its contents are displayed
instead of a diff. If the file is large, then the following hint is
inserted at the top:

  * Untracked file is 14774881 bytes.
  * Showing only first 131072 bytes.

Why exactly 131072 bytes? With this patch it is 100000 bytes.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>