Improve on how to get a branch name

Improve on how to get a branch name by reducing the frequency of executing shell commands.

I feel it's slow to execute a shell command **always twice** in order to get a branch name, and it's actually slow as far as I profiled.
So I reduce the frequency of executing shell commands referring to [oh-my-zsh's git plugin](//github.com/robbyrussell/oh-my-zsh/blob/master/lib/git.zsh#L4-L5).

### Profiling steps
1. Open a file in a clean git repository.
* <kbd>M-x elp-instrument-package RET helm-ls-git RET</kbd>
* <kbd>M-x elp-instrument-function RET shell-command-to-string RET</kbd>
* <kbd>M-x helm-ls-git-ls</kbd>
* <kbd>C-g</kbd> after `helm-ls-git-ls` results are displayed.
* <kbd>M-x elp-results</kdb>

### Profiling results
#### before
```
helm-ls-git-ls                         1           1.720472      1.720472
helm-ls-git-header-name                1           1.3683969999  1.3683969999
shell-command-to-string                2           1.367515      0.6837575
helm-ls-git-status                     1           0.008327      0.008327
helm-ls-git-init                       1           0.007876      0.007876
helm-ls-git-list-files                 1           0.007291      0.007291
helm-ls-git-root-dir                   3           0.000319      0.0001063333
helm-ls-git-transformer                1           0.000119      0.000119
helm-ls-git-status-transformer         1           6.4e-05       6.4e-05
helm-ls-git-sort-fn                    1           1.8e-05       1.8e-05
```

#### after
```
helm-ls-git-ls                         1           1.049938      1.049938
helm-ls-git-header-name                1           0.673281      0.673281
call-process-shell-command             1           0.673149      0.673149
helm-ls-git-status                     1           0.008407      0.008407
helm-ls-git-init                       1           0.006931      0.006931
helm-ls-git-list-files                 1           0.006734      0.006734
helm-ls-git-root-dir                   3           0.00026       8.666...e-05
helm-ls-git-transformer                1           0.000124      0.000124
helm-ls-git-status-transformer         1           6.4e-05       6.4e-05
helm-ls-git-sort-fn                    1           2.1e-05       2.1e-05
```