patch 8.1.0375: cannot use diff mode with Cygwin diff.exe

Problem:    Cannot use diff mode with Cygwin diff.exe. (Igor Forca)
Solution:   Skip over unrecognized lines in the diff output.

src/diff.c

@@ -1600,6 +1600,10 @@ diff_read(
 		    && (tag_fgets(linebuf, LBUFLEN, fd) == 0)
 		    && (STRNCMP(line, "@@ ", 3) == 0))
 		diffstyle = DIFF_UNIFIED;
+	    else
+		// Format not recognized yet, skip over this line.  Cygwin diff
+		// may put a warning at the start of the file.
+		continue;
 	}
 
 	if (diffstyle == DIFF_ED)

src/testdir/test_diffmode.vim

@@ -519,7 +519,9 @@ func Test_diffexpr()
   endif
 
   func DiffExpr()
-    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>' . v:fname_out
+    " Prepent some text to check diff type detection
+    call writefile(['warning', '  message'], v:fname_out)
+    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>>' . v:fname_out
   endfunc
   set diffexpr=DiffExpr()
   set diffopt=foldcolumn:0

src/version.c

@@ -794,6 +794,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    375,
 /**/
     374,
 /**/