git-gui: support for $FILENAMES in tool definitions
This adds a FILENAMES environment variable, which contains the repository
pathnames of all selected files the list.
The variable contains the names separated by LF (\n, \x0a).
If the file names contain LF characters, the tool command might be unable to
unambiguously split the value of $FILENAME into the separate names.
Note that the file marked and diffed immediately after starting the GUI up,
is not actually selected. One must click on it once to really select it.
Signed-off-by: Alex Riesen <alexander.riesen@cetitec.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>