fugitive: remove old fugitive test

As mentioned by @tpope, remove the old test for the autoloaded function
fugitivie#head() and instead use consistently FugitiveHead() everywhere

[delete] %bd command

[add] init.vimspec

[update] init.vimspec

[add] parts.vim

[add] section.vimspec

[add] themes.vimspec

[add] util.vimspec

[delete] vim-vspec

test/parts.vimspec

@@ -0,0 +1,58 @@
+Describe parts.vim
+
+  It overwrItes existing values
+    call airline#parts#define('foo', { 'test': '123' })
+    Assert Equals(airline#parts#get('foo').test, '123')
+    call airline#parts#define('foo', { 'test': '321' })
+    Assert Equals(airline#parts#get('foo').test, '321')
+  End
+
+  It can define a function part
+    call airline#parts#define_function('func', 'bar')
+    Assert Equals(airline#parts#get('func').function, 'bar')
+  End
+
+  It can define a text part
+    call airline#parts#define_text('text', 'bar')
+    Assert Equals(airline#parts#get('text').text, 'bar')
+  End
+
+  It can define a raw part
+    call airline#parts#define_raw('raw', 'bar')
+    Assert Equals(airline#parts#get('raw').raw, 'bar')
+  End
+
+  It can define a minwidth
+    call airline#parts#define_minwidth('mw', 123)
+    Assert Equals(airline#parts#get('mw').minwidth, 123)
+  End
+
+  It 'can define a condition'
+    call airline#parts#define_condition('part', '1')
+    Assert Equals(airline#parts#get('part').condition, '1')
+  End
+
+  It 'can define a accent'
+    call airline#parts#define_accent('part', 'red')
+    Assert Equals(airline#parts#get('part').accent, 'red')
+  End
+
+  It 'value should be blank'
+    Assert Equals(airline#parts#filetype(), '')
+  End
+
+  It 'can overwrIte a filetype'
+    set ft=aaa
+    Assert Equals(airline#parts#filetype(), 'aaa')
+  End
+
+  It 'can overwrite a filetype'
+    "GItHub actions's vim's column is smaller than 90
+    set ft=aaaa
+    if &columns >= 90
+      Assert Equals(airline#parts#filetype(), 'aaaa')
+    else
+      Assert Equals(airline#parts#filetype(), 'aaa…')
+    endif
+  End
+End