send-email: windows drive prefix (e.g. C:) appears only at the beginning

Tighten the regexp used in the "file_name_is_absolute" replacement
used on msys to declare that only "[a-zA-Z]:" that appear at the
very beginning is a path with a drive-prefix.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1118,7 +1118,7 @@ sub file_name_is_absolute {
 
 	# msys does not grok DOS drive-prefixes
 	if ($^O eq 'msys') {
-		return ($path =~ m#^/# || $path =~ m#[a-zA-Z]\:#)
+		return ($path =~ m#^/# || $path =~ m#^[a-zA-Z]\:#)
 	}
 
 	require File::Spec::Functions;