Use git describe to get the HOMEBREW_VERSION.

For tagged commits produces the output:
- `1.0.1`

For untagged commits with a dirty tree produces the output:
- `1.0.1-19-g23efbc5-dirty`

Performance:
```
git describe --tags --dirty 2> /dev/null
0.07s user 0.01s system 96% cpu 0.086 total
```

This means we can tag any commit without needing to manually remember
to bump the revision every time.

1 files changed, 1 insertions, 1 deletions

diff --git a/Library/Homebrew/brew.rb b/Library/Homebrew/brew.rb
index eeadc70e1..0d1bcbdda 100644
--- a/Library/Homebrew/brew.rb
+++ b/Library/Homebrew/brew.rb
@@ -14,7 +14,7 @@ $:.unshift(HOMEBREW_LIBRARY_PATH.to_s)
 require "global"
 
 if ARGV == %w[--version] || ARGV == %w[-v]
-  puts "Homebrew #{Homebrew.homebrew_version_string}"
+  puts "Homebrew #{HOMEBREW_VERSION}"
   puts "Homebrew/homebrew-core #{Homebrew.core_tap_version_string}"
   exit 0
 end