cache location of session file

Function session_file() returns the name of session file.
This function is called for every change in VIM layout.

Caching of return variable of this function gives x100 performance
improvement.

Before change:
FUNCTIONS SORTED ON TOTAL TIME
count  total (s)   self (s)  function
    6   3.369383   0.015592  g:GitSessionUpdate()
    6   3.353791   0.000861  <SNR>35_session_file()
    6   3.268404   0.001057  <SNR>35_session_dir()
   12   3.263580   0.004735  <SNR>35_in_git_repo()

After this change:
FUNCTIONS SORTED ON TOTAL TIME
count  total (s)   self (s)  function
   11   0.026195             g:GitSessionUpdate()
   23   0.025646             <SNR>38_Highlight_Matching_Pair()
   11   0.000956             nerdtree#checkForBrowse()

VIM profiling (http://stackoverflow.com/a/12216578):
	:profile start profile.log
	:profile func *
	:profile file *
	" At this point do slow actions
	:profile pause
	:noautocmd qall!

Signed-off-by: Leon Romanovsky <leon@leon.nu>

plugin/gitsessions.vim

@@ -109,7 +109,6 @@ endfunction
 
 function! g:GitSessionSave()
     let l:dir = s:session_dir()
-    let l:file = s:session_file()
 
     if !isdirectory(l:dir)
         call mkdir(l:dir, 'p')
@@ -126,24 +125,23 @@ function! g:GitSessionSave()
     endif
 
     let s:session_exist = 1
-    if filereadable(l:file)
-        execute 'mksession!' l:file
-        echom "session updated:" l:file
+    if filereadable(s:s_file)
+        execute 'mksession!' s:s_file
+        echom "session updated:" s:s_file
     else
-        execute 'mksession!' l:file
-        echom "session saved:" l:file
+        execute 'mksession!' s:s_file
+        echom "session saved:" s:s_file
     endif
     redrawstatus!
 endfunction
 
 function! g:GitSessionUpdate(...)
     let l:show_msg = a:0 > 0 ? a:1 : 1
-    let l:file = s:session_file()
 
-    if s:session_exist && filereadable(l:file)
-        execute 'mksession!' l:file
+    if s:session_exist && filereadable(s:s_file)
+        execute 'mksession!' s:s_file
         if l:show_msg
-            echom "session updated:" l:file
+            echom "session updated:" s:s_file
         endif
     endif
 endfunction
@@ -154,32 +152,35 @@ function! g:GitSessionLoad(...)
     endif
 
     let l:show_msg = a:0 > 0 ? a:1 : 0
-    let l:file = s:session_file()
 
-    if filereadable(l:file)
+    if filereadable(s:s_file)
         let s:session_exist = 1
-        execute 'source' l:file
-        echom "session loaded:" l:file
+        execute 'source' s:s_file
+        echom "session loaded:" s:s_file
     elseif l:show_msg
-        echom "session not found:" l:file
+        echom "session not found:" s:s_file
     endif
     redrawstatus!
 endfunction
 
 function! g:GitSessionDelete()
-    let l:file = s:session_file()
     let s:session_exist = 0
-    if filereadable(l:file)
-        call delete(l:file)
-        echom "session deleted:" l:file
+    if filereadable(s:s_file)
+        call delete(s:s_file)
+        echom "session deleted:" s:s_file
     endif
 endfunction
 
+function! g:GitCacheVariables()
+     let s:s_file = s:session_file()
+endfunction
+
 augroup gitsessions
     autocmd!
     if ! exists("g:gitsessions_disable_auto_load")
         autocmd VimEnter * :call g:GitSessionLoad()
     endif
+    call g:GitCacheVariables()
     autocmd BufEnter * :call g:GitSessionUpdate(0)
     autocmd VimLeave * :call g:GitSessionUpdate()
 augroup END