Add mapping to yank current commit hash

As you can't enter a Vim popup window, you can't easily yank the current
commit hash, and that seems a natural thing to want to do (I certainly
found myself wanting to do it soon after starting to use git-messenger).

To address this, map 'c' to yank the current commit hash to v:register
(kind of, while the code behaves the same as using v:register, it
doesn't actually use v:register due to some clash with vim-cutlass).

While this is added primarily to aid Vim popup users, it doesn't break
anything for Neovim floating win or preview window users as the buffer
is not modifiable, and it seems a handy mapping for all users anyway.

Author: mmrwoods

README.md

@@ -139,6 +139,7 @@ Following mappings are defined within popup window.
 | `D`     | Toggle all unified diff hunks of the commit                  |
 | `r`     | Toggle word diff hunks only in current file of the commit    |
 | `R`     | Toggle all word diff hunks of current commit                 |
+| `c`     | Yank/copy the current commit hash to `v:register`            |
 | `?`     | Show mappings help                                           |
 
 ### Mappings

autoload/gitmessenger/blame.vim

@@ -63,6 +63,22 @@ function! s:blame__forward() dict abort
 endfunction
 let s:blame.forward = funcref('s:blame__forward')
 
+function! s:blame__yank_hash() dict abort
+    " Note: v:register is blackhole here when vim-cutlass plugin used
+    " TODO: investigate further, it should be possible to use v:register
+    let register = '"'
+    if has('clipboard')
+        if stridx(&clipboard, 'unnamedplus') != -1
+            let register = '+'
+        elseif stridx(&clipboard, 'unnamed') != -1
+            let register = '*'
+        endif
+    endif
+    call setreg(register, self.state.commit)
+    echo 'git-messenger: yanked commit hash ' . self.state.commit
+endfunction
+let s:blame.yank_hash = funcref('s:blame__yank_hash')
+
 function! s:blame__open_popup() dict abort
     if has_key(self, 'popup') && has_key(self.popup, 'bufnr')
         " Already popup is open. It means that now older commit is showing up.
@@ -80,6 +96,7 @@ function! s:blame__open_popup() dict abort
         \       'q': [{-> execute('close', '')}, 'Close popup window'],
         \       'o': [funcref(self.back, [], self), 'Back to older commit'],
         \       'O': [funcref(self.forward, [], self), 'Forward to newer commit'],
+        \       'c': [funcref(self.yank_hash, [], self), 'Yank/copy current commit hash'],
         \       'd': [funcref(self.reveal_diff, [v:false, v:false], self), "Toggle current file's diffs"],
         \       'D': [funcref(self.reveal_diff, [v:true, v:false], self), 'Toggle all diffs'],
         \       'r': [funcref(self.reveal_diff, [v:false, v:true], self), "Toggle current file's word diffs"],

doc/git-messenger.txt

@@ -137,6 +137,7 @@ COMMANDS                                                *git-messenger-commands*
   |    D    | Toggle all unified diff hunks of the commit                  |
   |    r    | Toggle word diff hunks only in current file of the commit    |
   |    R    | Toggle all word diff hunks of the commit                     |
+  |    c    | Yank/copy the current commit hash to |v:register|              |
   |    ?    | Show mappings help                                           |
 
 *:GitMessengerClose*