Escape characters from tags that have special meaning in grep.

In particular, this fixes the case where a dot in a version name (not
too uncommon ;)) unintentionally matches a pre-existing tag.  For
example, if these tags exist:

    1.0.1
    1.0b2

And you try to start a new release for 1.0.2, git-flow prevented it,
since the '.' matches the 'b' in 1.0b2.  This resulted in the invalid
error message:

    Tag '1.0.2' already exists. Pick another name.

gitflow-common

@@ -44,10 +44,14 @@
 warn() { echo "$@" >&2; }
 die() { warn "$@"; exit 1; }
 
+escape() {
+	echo "$1" | sed 's/\([\.\+\$\*]\)/\\\1/g'
+}
+
 # set logic
 has() {
 	local item=$1; shift
-	echo " $@ " | grep -q " $item "
+	echo " $@ " | grep -q " $(escape $item) "
 }
 
 # basic math
@@ -211,7 +215,7 @@ gitflow_resolve_nameprefix() {
 		return 0
 	fi
 
-	matches=$(echo "$(git_local_branches)" | grep "^$prefix$name")
+	matches=$(echo "$(git_local_branches)" | grep "^$(escape "$prefix$name")")
 	num_matches=$(echo "$matches" | wc -l)
 	if [ -z "$matches" ]; then
 		# no prefix match, so take it literally