git-gui: Brown paper bag "dirty git version fix"

My prior change to allow git-gui to run with a version of Git
that was built from a working directory that had uncommitted
changes didn't account for the pattern starting with -, and
that confused Tcl.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/git-gui.sh b/git-gui.sh
index bdb5571..3808451 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -544,7 +544,7 @@
 	error_popup "Cannot parse Git version string:\n\n$_git_version"
 	exit 1
 }
-regsub {-dirty$} $_git_version {} _git_version
+regsub -- {-dirty$} $_git_version {} _git_version
 regsub {\.[0-9]+\.g[0-9a-f]+$} $_git_version {} _git_version
 regsub {\.rc[0-9]+$} $_git_version {} _git_version