git-browser.cgi: use --stdin option when running rev-list

git-browser.cgi: use --stdin option when running rev-list

There can potentially be a substantial number of arguments that need
to be passed to the `git rev-list` command.  However the number of
arguments that can be passed on the command line may be limited --
severely so on some platforms.

Fortunately, git has provided for the eventuality with the `--stdin`
option to the `rev-list` command which allows the arguments to be
passed in via standard input.

Each argument must appear on an input line by itself and is either
a ref or, after seeing '--' on a line by itself, a path spec.  No
options (such as `--not`) are allowed via the `--stdin` input.

Since exclusion refs were recently converted to use the `^` prefix
instead of the `--not` option, the switchover to using `--stdin`
becomes rather simple as long as care is taken to avoid being
bitten by a SIGPIPE while writing the input arguments.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>