Checks if git is installed

When sources are to be fetched using git, check beforehand it is available
in the path instead of causing an error.

It will show the message:

`Error: 'git' program not found. Is git installed? You may want to edit variables.GIT`

and exit.

refs #381

1 files changed, 4 insertions, 0 deletions

diff --git a/src/luarocks/fetch/git.lua b/src/luarocks/fetch/git.lua
index 53fd4445..bea53fc8 100644
--- a/src/luarocks/fetch/git.lua
+++ b/src/luarocks/fetch/git.lua
@@ -40,6 +40,10 @@ function git.get_sources(rockspec, extract, dest_dir, depth)
    -- Strip off .git from base name if present
    module = module:gsub("%.git$", "")
 
+   if not fs.execute_quiet(git_cmd, "--version") then
+      return nil, "'"..git_cmd.."' program not found. Is git installed? You may want to edit variables.GIT"
+   end
+
    local store_dir
    if not dest_dir then
       store_dir = fs.make_temp_dir(name_version)