Fix hashes when awk does not support interval expressions (#18)

Improve compatibility with more awk implementations like mawk or nawk by
changing interval expression (not supported in those variants) with
repetition. Even gawk which does support interval expressions, required
additional option to enable such support until version 4.0.

Fixes #17

fzf-git.sh

@@ -180,7 +180,7 @@ _fzf_git_hashes() {
     --bind 'ctrl-d:execute:grep -o "[a-f0-9]\{7,\}" <<< {} | head -n 1 | xargs git diff > /dev/tty' \
     --color hl:underline,hl+:underline \
     --preview 'grep -o "[a-f0-9]\{7,\}" <<< {} | head -n 1 | xargs git show --color=always' "$@" |
-  awk 'match($0, /[a-f0-9]{7,}/) { print substr($0, RSTART, RLENGTH) }'
+  awk 'match($0, /[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]*/) { print substr($0, RSTART, RLENGTH) }'
 }
 
 _fzf_git_remotes() {