fix: check if git is installed

[dimitry-ishenko]

Without git installed, I get the following error:

Error detected while processing /home/dimitry/.config/nvim/plugin/50-lualine.lua:
E5113: Error while calling lua chunk: Vim:E475: Invalid value for argument cmd: 'git' is not executable
stack traceback:
        [C]: in function 'jobstart'
        ...pack/dist-bundle/start/lualine/lua/lualine/utils/job.lua:9: in function 'start'
        ...e/start/lualine/lua/lualine/components/diff/git_diff.lua:141: in function 'update_git_diff'
        ...e/start/lualine/lua/lualine/components/diff/git_diff.lua:129: in function 'update_diff_args'
        ...e/start/lualine/lua/lualine/components/diff/git_diff.lua:28: in function 'init'
        ...undle/start/lualine/lua/lualine/components/diff/init.lua:57: in function 'init'
        ...ck/dist-bundle/start/lualine/lua/lualine/utils/class.lua:34: in function 'loaded_component'
        ...k/dist-bundle/start/lualine/lua/lualine/utils/loader.lua:30: in function 'mod'
        ...k/dist-bundle/start/lualine/lua/lualine/utils/loader.lua:79: in function 'component_loader'
        ...k/dist-bundle/start/lualine/lua/lualine/utils/loader.lua:148: in function 'load_sections'
        ...k/dist-bundle/start/lualine/lua/lualine/utils/loader.lua:159: in function 'load_components'
        ...k/dist-bundle/start/lualine/lua/lualine/utils/loader.lua:204: in function 'load_all'
        ...nvim/site/pack/dist-bundle/start/lualine/lua/lualine.lua:690: in function 'setup'
        /home/dimitry/.config/nvim/plugin/50-lualine.lua:1: in main chunk
Error detected while processing BufEnter Autocommands for "*":
E5108: Error executing lua Vim:E475: Invalid value for argument cmd: 'git' is not executable
stack traceback:
        [C]: in function 'jobstart'
        ...pack/dist-bundle/start/lualine/lua/lualine/utils/job.lua:9: in function 'start'
        ...e/start/lualine/lua/lualine/components/diff/git_diff.lua:141: in function 'update_git_diff'
        ...e/start/lualine/lua/lualine/components/diff/git_diff.lua:129: in function 'update_diff_args'
        [string ":lua"]:1: in main chunk
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